It is almost always images stored at far higher resolution than the slide displays them at, plus media left behind on deleted slides. Find which files account for the size, replace or remove those, and a deck typically drops by most of its weight.
Open this small example in the file size analyzer. Compare the media entries with the slide XML, then open the presentation and find the illustration. This demonstrates how to identify stored assets; it is not a compression benchmark or a promise of a particular saving.
Twenty slides, mostly text, and the file is 80 MB. Email rejects it, the shared drive complains, and nothing on screen looks like it should weigh that much.
Slides are cheap. The text in a whole deck is a few kilobytes. Something else is taking the space, and it is worth finding out precisely what rather than guessing.
Find the actual culprit first
A .pptx is a ZIP archive. Its parts can be listed and measured, which turns "the file is too big" into "this one photograph is 34 MB". That changes the fix from a blind compression pass into a targeted one.
In nearly every oversized deck, a handful of images account for almost all of it.
Why images are so much larger than they look
Resolution has nothing to do with display size
Drag a 6000-pixel-wide photograph from a phone onto a slide and shrink it to fill a quarter of the screen. PowerPoint stores all 6000 pixels. The slide displays perhaps 800 of them. The other 5200 are carried in the file forever, doing nothing.
Cropping usually hides rather than removes
Cropping normally keeps the whole original so the crop stays adjustable. A tightly cropped detail from a huge photograph can still be storing the huge photograph — which is both a size problem and, occasionally, a confidentiality one.
Deleted slides leave their media behind
Media referenced by a deleted slide, or living on an unused slide master, often stays in the archive. Decks that have been edited over years accumulate this quietly.
PNG is lossless, which is right for screenshots of interfaces and text. For a photograph it is entirely the wrong format and can be ten times the size of an equivalent JPEG. A screenshot that happens to contain a photograph gets the worst of both.
Fixing it
Use PowerPoint’s own compression
Select a picture, then Picture Format → Compress Pictures. Untick "Apply only to this picture", choose a sensible resolution, and tick "Delete cropped areas of pictures". That last option is the one that recovers the most, and it is the one people miss.
150 ppi is ample for a deck that will be presented or emailed. 220 ppi if it will be printed.
Resize the worst offenders before inserting them
For a deck you are still building, resizing images to roughly the size they will display — and converting photographs to JPEG or WebP — prevents the problem rather than repairing it. Batch image conversion handles a folder in one pass.
Send fewer slides
If the recipient needs six slides out of forty, sending six is better in every way. Selecting slides produces a trimmed deck that still carries the layouts and media those slides depend on, so it opens as a proper presentation rather than a broken one.
Send a PDF instead
If the recipient only needs to read it, converting to PDF may be useful. Compare file sizes and inspect every slide before sending: browser conversion reconstructs the layout, and custom fonts, charts or unsupported objects can change. Keep the original presentation for editing.
The email limit is smaller than it says
A working order
Analyse the file and identify which parts account for the size.
Compress pictures across the whole deck, with cropped areas deleted.
Remove slides the recipient does not need.
Re-check the size. Most decks are done by here.
If it is still too large, send a PDF or a link instead.
Keep the original until you have checked the compressed version on a large screen. Image compression is irreversible, and a photograph that looked fine in a thumbnail can be visibly degraded when projected.