Why a video got bigger after you converted it on a Mac

25 September 2026 · 6 min read

Converting a video changes how it is stored; nothing about conversion promises a smaller file. When the result comes out bigger, it is almost always one of four things: the new format is less efficient than the old one, the converter was told to spend more data than the original did, the picture was enlarged or its frame rate raised, or the audio was stored uncompressed. None of them adds quality. The extra megabytes describe the same picture less economically.

The fix is to find which one happened, go back to the original, and convert once with settings that match it.

Nothing you convert gains detail

This shapes every fix below, so it is worth saying first. A converter decodes the original into plain frames and then compresses them again. Whatever the original lost when it was first recorded is gone, and the new file cannot bring it back. A bigger copy holds the same detail, sometimes with a fresh layer of compression blemishes on top.

So a bigger converted file is not a higher quality file. It is the same picture at a higher price. Why compressing a video twice makes it look worse explains what each extra encode costs.

Find out what changed

QuickTime Player can tell you in a few seconds. Open the original, choose Window > Show Movie Inspector (Command-I), and note what it lists: the video format, the resolution, the frame rate, the audio format and the data rate. Open the converted copy and do the same. The difference between the two panels usually names the cause directly.

If QuickTime cannot open one of the files, which is common with MKV or WebM originals, compare what you can: resolution in Finder’s Get Info, and the file sizes against the lengths.

Reason 1: the new codec is less efficient

This is the most common one on a Mac. Recent iPhones record in HEVC (H.265), which was designed to reach the same look as H.264 at a lower bitrate. Convert an iPhone video to H.264 MP4 so an older device or a colleague can play it, which is often the right call, and the encoder needs more data to hold the same picture. The file grows.

The same happens whenever you move from a newer codec to an older one: VP9 or AV1 from a web download converted to H.264, or anything converted to an older format for an old TV or player.

If the Movie Inspector shows HEVC for the original and H.264 for the copy, you have your answer. The growth is the price of compatibility. Whether that price is worth paying depends on who needs to open the file; H.264 vs HEVC goes through who can play which.

Reason 2: the converter spent more than the original did

Every encoder aims for a quality level or a bitrate. If the converter was set to its highest quality preset, or to a fixed bitrate above the original’s, it spends more data per second than the source did, even when the codec stays the same. A phone video recorded at a moderate bitrate and converted at maximum quality comes out larger with no visible benefit.

Compare the data rates in the two Movie Inspector panels. If the codec is the same and the copy’s rate is well above the original’s, the conversion was set too generously.

Some conversions need not touch the picture at all. Moving H.264 video from an MKV into an MP4, for example, can copy the frames across unchanged, keeping the size almost identical and costing no quality. Remux versus re-encode explains when that is possible.

Reason 3: the picture got larger or faster

A 1080p video exported at 4K has four times as many pixels in every frame, and the encoder pays for all of them even though the extra ones hold no new detail. A 30 frames per second video converted to 60 frames per second doubles the frames, and the new ones are copies or blends of their neighbors, paid for in full.

In the Movie Inspector, compare the resolution and FPS lines. If either went up, export again at the source’s own resolution and frame rate, or lower. When an export dialog offers a resolution larger than the original, choosing it is how this happens.

Reason 4: the audio was stored uncompressed

Most MP4 and MOV files carry compressed audio, usually AAC. Some conversions, particularly to formats aimed at editing, store the sound as uncompressed PCM instead. On a long recording that adds real weight. The Movie Inspector names the audio format; Linear PCM means uncompressed.

The same principle explains audio-only conversions that balloon. WAV at CD quality runs at about 1,411 kilobits per second, many times the rate of a typical MP3, because it stores the decoded samples as they are. Converting an MP3 to WAV makes it far bigger and no better. Lossless vs lossy covers why.

Images do this too

Photos follow the same rules. A JPEG converted to PNG usually grows considerably, because PNG stores every pixel of the decoded image exactly, including the blemishes the JPEG compression put there. A HEIC converted to JPEG grows for the same reason an HEVC video does. And an animated GIF made from a video is the extreme case; why a GIF is bigger than the video it came from explains that one.

Getting the size back down

Start by deciding what the conversion was for.

If it was for compatibility, keep the conversion but control it. Go back to the original rather than the converted copy, and convert once at the original’s resolution and frame rate, with compressed audio.
If the result has to fit a limit, such as an email attachment or an upload form, set the size directly instead of guessing at settings.
If there was no particular reason, keep the original and delete the bigger copy. The original is the better file.

Manatee handles the first two. Convert mode takes the original and writes an MP4 or MOV beside it. Shrink mode takes a video and a size in MB and finds the quality that fits in a single encode, again as a new file with the original untouched. When size is the goal, give Shrink the original rather than a copy that was already converted, so the picture is compressed once instead of twice. If the recipient’s device was the reason for converting in the first place, play the result on that device before deleting anything. Everything runs on your Mac, so nothing is uploaded along the way.

Questions

Is the bigger file better quality? No. It can match the original at best. Detail lost in the first recording cannot be restored by any later conversion.

Why did exporting from QuickTime Player make my iPhone video bigger? Most likely because the export used H.264 on an HEVC original. When QuickTime Player’s Export As asks whether you want H.264 for compatibility or HEVC for a smaller file, choose HEVC if the recipient can play it.

Can I just shrink the big converted copy? You can, but it adds another round of compression to a file that has already had one extra. Starting from the original gives a better picture at the same size.