thank you for your awesome reply. I'm no expert, either, so every bit of information is helpful. I'm actually blown away by the scope of your work.
It makes sense to drop your images down to dimensions of 1024x768 if you're not expecting your readers to zoom (why don't you expect them to zoom?). Image quality in the epubs as loaded (not zoomed in) is good. I'm sure they would retain that quality on your iPad Pro. It's just the zooming that creates issues.
Kobo's not the only retailer with a 100MB limit, and the need to retain some level of quality at zoomed-in views makes 100MB a real hurdle. Mind you, I'm pleased that the epubs I'm creating for the current project are below that limit (though not by much) while delivering okay zoomed-in visuals. Not perfect visuals, but okay.
I've just checked, and in Pubreader I can't zoom in on pages either. I can't remember if earlier versions of Pubreader used to zoom in or not.
I need to revisit my Android tablet regarding zoom. I think I could zoom-in in the Kobo app, on a Samsung tablet. I've been focusing first on a version for Apple Books this time round. Android is next up. But if you're creating Android apps (as opposed to epubs), I don't think they would zoom because the Pubcoder apps are basically the Xpub file wrapped in a different shell. And it seems the xpubs don't zoom ...
You're right that a lot of readers are using Android. I had tracking links set up in a free ebook to see where hits were coming from, and there were big numbers coming from Google Play books. Not as many as I was getting from Kindle apps and devices, but close. But when I delved into the app-creation side of Pubcoder, that was a whole other thing in itself. I built an iOS app (great agony and suffering involved) and stopped. I'm so impressed that you've created apps for both platforms and haven't been driven mad :)
We've moved in opposite directions, I think. I've come to believe that iOS is the best - perhaps the only - platform for my interactive eBooks, but I actually started designing for Kindle. I hand-coded an interactive edition for Kindle, and it worked, but Kindle only supports little bits of the EPUB3 spec, and does it in proprietary ways; and what I could achieve on that platform could not compare with a proper EPUB3 implementation (via Pubcoder) on iOS. I'm actually thinking of going back to flat-file picture books on the other platforms with, as you say, text big enough to read without zooming. My first picture book was a flat file, and was only about 10MB.
The interactive books are big files. There's no way round it, I think. And unfortunately, Apple Books doesn't seem to attract the numbers of readers who use the other platforms, like Kindle and Android ... and Kindle devices are just Android devices with Amazon-proprietary limitations built in.
Again, thanks for sharing so much info. I would love to see what you're working on. Can I buy your books?