All sorts can happen between source and binary.
1. The probability of discovering something in all that binary code, especially with the intricate and non-orthogonal nature of x86/x64 assembly and odd compiler optimisations. This isn't some 80's game.
A comparison:
28500000 = Chrome binary size [1]
750000000 = Human Genome size (converted to bytes - 1BP = 2bits so 4BP per byte) [2]
2. The probability of a vulnerability being published to Google versus selling it on the private market.
True. But also true of firefox. But you can install chromium and firefox from source and be sure that apart from your compiler nobody planted anything in your browser.