Now you're in my wheelhouse.

Message:

JPG can compress much more than PNG. Period...

Main advantages of PNG are lossless encoding and support for transparency. PNG is used when you need to maintain image fidelity (or require an alpha channel) for 8-bit/pixel images.

JPG for when you want to cram an image.

Red - Picture Format tab in Excel. Adjust group. Compress Pictures. Here you can crank up the compression and get your file size down...

Because JPEG is LOSSY it throws away image detail. Recompressing a JPEG is throwing lossy compression on an already damaged image. Eventually you'll get bad artifacts. I'll betcha by default Excel uses minimal compression to maintain image fidelity and avoid image degradation.

All the above also applies to MPEG video compression, including h.264/mp4 and h.265/mp4 - which is why intermediate codecs like ProRes and DNXHD are a thing.

Bottom line - lossless image/video compression can get you a 2:1-4:1 compression ratio. Lossy compression can get you well over 100:1. But the smaller you cram the file, the worse it looks.

Yours,