It's nothing to do with how much space it will take up. The problem is that, as a user, I would be very suspicious of a 100MB countdown timer app. I would think that either it's got some kind of adware in it, or just that it's incompetently designed.

My laptop has a 256GB SSD. My media collection is 337GB. So a 100MB app doesn't just need to compete with other 100MB apps, it has to compete with my music, videos, and photos. Is a 100MB app better than my photos reminding me of my time in Sapporo, Japan? That depends on the app, but I'm more inclined to delete a 100MB app to make room for more photos than a 20MB app.