Before we shipped video exports we wrote down the reasons not to. The main one was size. Our own planning document put a five-slide carousel video at about 8 MB, and at that number a flat monthly price starts to look uncomfortable.
Then we measured a real one. It came out at 165 KB — roughly a third the size of the five PNG slides it was made from.
We were wrong by about forty times, in the direction that killed the objection. That estimate had been sitting in a document for weeks looking like a fact. It nearly stopped the feature before anyone tried it.
The reason it was so wrong is the useful part, because it tells you which of your videos will be a problem.
File size is about change, not about length
The intuition most people have is that a video's size comes from how long it is and how big the picture is. Both matter a little. Neither is the main thing.
What actually drives it is how much changes between one frame and the next.
A video codec does not store every frame as a picture. It stores a few real frames, and between them it stores the differences. If almost nothing moved, the difference is almost nothing, and that stretch of video costs almost no space.
A carousel video is the extreme case. Each slide is a still image held for a few seconds. For most of its running time, nothing changes at all — the codec stores "same as before" over and over, which is nearly free. The whole file is five pictures and some instructions about when to swap them.
Which is why your phone footage is enormous
Now point a phone at a street.
Every frame is different from the last. The camera is moving slightly even when you think it is still, every leaf and pedestrian is moving independently, the autofocus is breathing, and the sensor adds a layer of noise that is different random noise every frame. The codec cannot say "same as before" about any of it.
Three things make a video genuinely large, in rough order:
Motion and noise. Handheld footage, crowds, foliage, water, rain, confetti. Grain from a low-light sensor is the worst case, because random noise is incompressible by definition — it is the one thing that never looks like what came before.
A high bitrate your camera chose for you. Modern phones shoot 4K at 50 Mbps or more because storage is cheap on the device. That is a deliberate quality setting, not a law of physics, and most of it is invisible once the clip is 400 pixels wide in a feed.
Length, last. Doubling the duration does roughly double the size — but it is a multiplier on a number the first two already decided.
Resolution matters less than people expect. A static 4K title card is small. A noisy 720p night clip can be larger than a clean 4K one of a blank wall.
What this means for posting
Anything PostPop exports is already small. Slides, carousels, and the Motion videos made from them. You will not hit a platform limit with one and there is nothing to do.
Footage you shot is the thing to watch. A couple of minutes of handheld 4K can be 500 MB, and that is where you meet a limit: Discord's free tier at 10 MB, email attachments around 25 MB, a WhatsApp send capped around 16 MB, or a platform upload that simply times out on a hotel connection.
Compress before you upload, not after. Every platform re-encodes what you give it. Sending a 500 MB file so the platform can shrink it to 8 MB on its own terms wastes your upload and gives you no say in what gets thrown away. Shrinking it yourself first means you choose where the quality goes.
What to use
For the case above — a clip that is too big to send — SquishyFile does it in the browser, and the part worth knowing is that the video never leaves your device. It compresses on your own machine rather than uploading to a server, so there is no queue, no account, and no copy of your footage sitting somewhere. You can also give it a target — "under 16 MB" — instead of guessing at a quality slider.
That on-device detail is the reason we are comfortable pointing at it. Our own safe-zone checker works the same way and never uploads your image either, and we would look silly recommending a tool that does the opposite.
The wider point
The 8 MB figure was not a lie. It was a guess, written down, that stopped looking like a guess after a few weeks in a document.
That is worth watching for in your own work. A number nobody has measured, sitting in a plan where everyone can see it, quietly becomes a constraint — and in our case it nearly cost a feature that turned out to be forty times cheaper than we thought.
The fix is boring and it is the same every time: make one, and weigh it.