Why a photo in the document weighs more than on disk
When you insert an image into Word or PowerPoint, the complete original file is embedded into the document — at full resolution, even if it appears only stamp-sized on the slide. An 8 MB phone photo stays an 8 MB photo, even if you drag it to thumbnail size afterward: PowerPoint just displays it smaller, but all pixels stay stored. Ten such photos, and the file is at 80 MB.
On top of that comes a catch that surprises many: cropping doesn't delete the cut-away parts. Crop an image in PowerPoint to a detail, and the file keeps the whole photo — the rest is only hidden so you can change the crop later. Show a small crop from a big photo, and you drag the full file along.
Way 1: the built-in compression (fast, right in Office)
Both programs bring an image compression that solves exactly these problems:
- Click an image → the Picture Format (or Graphics Format) tab appears in the ribbon → there Compress Pictures.
- In the dialog choose a resolution — for on-screen presentations and PDF sending, the lower web/email level (about 150 ppi) is plenty; the print level only if the file is really printed.
- Important checkbox: enable "Delete cropped areas of pictures" — that throws out the hidden crop remnants. And uncheck "Apply only to this picture" so the setting applies to all images in the document.
That alone often brings an 80 MB presentation to under 10 MB. One caveat: the operation is lossy and not reversible — so save a copy of the file beforehand in case full resolution is needed after all.
Way 2: shrink images before inserting (the cleaner way)
Even better than cleaning up afterward: don't bloat in the first place. If you know images are going into a document, shrink them beforehand to a sensible size — that keeps the file lean from the start and gives more control over the quality than the Office automatism:
- Batch the photos through the resize tool to the size actually needed. Ballpark: a full-slide image ~1920 px wide, half a slide ~1000 px, a small inline image in Word ~800 px.
- Gain a bit more with JPG compression (quality 80) — but for diagrams and screenshots with sharp edges, better PNG, or the text frays.
- Insert only these lean versions. Everything runs browser-local, the images don't leave your computer.
Screenshots: the most common single culprit
Strikingly often it's not photos but screenshots that bloat Office files — especially from high-resolution monitors, where a single screenshot is 4–8 megapixels. For the slide, rarely more than 1920 px in width is needed. How to get screenshots small yet sharp (and why JPG is often the wrong choice here) is in Optimizing screenshots.
The hidden size drivers besides images
Usually it's the images — but not always. If the file is still big after image compression, look at:
- Embedded videos: one clip in the PowerPoint quickly weighs more than all images combined — better to link than embed.
- Embedded fonts: the "Embed fonts in the file" option adds complete font families; only needed for other people's presentation computers.
- Old versions / fast saving: occasionally doing a "Save As" with a new name instead of endless overwriting clears accumulated ballast.
In short
Office embeds images at full size and even keeps what you cropped away. The quick fix: built-in image compression with "delete cropped areas". The better way: bring images to the needed size before inserting. And if you send the file by mail, combine it with the tips from Email attachment too big.
Sources
Microsoft — Reduce a picture's file size in Office · Microsoft — Office support.