Proxy images for linked vector/type PDFs

So: I have a 200-page document with 5 chapters plus front matter, and each chapter has in the neighborhood of 1000 linked images. The vast majority of these images are type-only PDFs (what raster images there are have been optimized for the publication). The linked PDFs are all of individual signs (one chapter per language), which vary in size from about 4"x4" to 85"x9", though most are on the smaller end of that, say 10"x2.5", and 11"x7". Most are placed at around 15-20%. The InDesign files for this (without the links) is 2.27GB. I can get a low-res PDF with bookmarks down to 16MB after optimization, though it's nearly 900MB after export.

All this is working well enough for the client, I'm just wondering if there's a way I can cut down on the InDesign file size. I'm pretty sure it comes down to InDesign's proxy images. The reason is that I can save .idml files, and they get very small, but as soon as I save again as .indd, the file size goes back up to what it was. My Display Performance preferences are set to "typical," in the middle setting. There are only 3 choices: grayed out, typical/proxy, and high resolution. I need to be able to see the content, so they can't be grayed out. (It gets edited every quarter, signs removed, new signs added, and they have to be displayed in alphabetical order.)

Is there anything I can do, or are these file sizes reasonable? Is there some other place I can control how InDesign is generating the proxy images?