(I'm the Flutter TL.)
Yeah, I was just writing a design doc earlier today and my math for how big a download we could reasonably expect users to wait for was based on a 5Mbit/s download speed. Half a megabyte takes under a second at that speed, but 5 megabytes takes more like 8 seconds. That's a huge difference. In an environment like the Web, where ephemerality is the norm, you really have to stay below 1MB from what I can tell.