A 40-slide deck that is 90 MB is 90 MB of pictures. Titles, bullets and shapes cost almost nothing; the photos, screenshots and exported charts do. That has two consequences. First, the fix is per-image, not per-file: every slide tool either re-compresses embedded images on save or leaves them untouched, and neither of those is a size plan you control. Second, the images have a frame to fit. A standard widescreen deck is 16:9 (1920 x 1080 px is the common export size), an older deck is 4:3 (1600 x 1200 px), and pasting a 6000 px camera photo into either one just gives the tool a bigger number to store. Compressing to a byte target and resizing to the frame are separate controls, and you usually want both: resize down to the frame, then compress until each image is a few hundred KB. Skwish does both in the browser with a before and after per file, so you can pick a target and stay consistent across the whole deck.