High Rez PDFs shouldn't have such large file sizes
If I layout the same design in Illustrator AND InDesign, Illustrator will always have a ridiculously higher file size. InDesign will produce a print-ready PDF at 9 megs, and Illustrator will produce one that is 34 megs which can't be sent by email. I've tried rasterizing photos and cropping them but this is a rather arduous process and has resulted in a lot of errors as the rasterize process isn't great either.
When a PDF-file 'saved' in Illustrator, it gets an internal copy of the Ai-document written in it, so that users can always read it back in to make changes. This includes all your stuff on canvas, all presets, brushes, styles, swatches... it’s all compact enough, but even without images it makes a PDF twice the size. Once images jump in, it rockets sky high — obviously enough.
InDesign does not care about editability of PDFs, it 'exports' PDF, with no chance of them being read back in a way and form they were created — that’s why 'save' and 'export' are in quotation marks.
If you want to make a PDF smaller in Illustrator — just use correct presets and/or settings while 'saving' it. Choose 'Smallest File Size' preset, make sure you have 'Preserve Illustrator Editing Capabilities' option unchecked, tweak compression, resampling and image quality.
Read more about making PDFs here: