So I was mining in extreme hills biome, with a standard 2x1 tunnel approach, meaning all the gaps I have around are 1 block wide. When I break monster egg blocks, silverfish do not spawn at the location of the broken block, but below it. This causes most silverfish to suffocate in the stone.

I've set up an experiment in creative mode, setting up a 2 blocks stone high monster eggs, surrounded by stained clay in all 8 directions. Looks like this from the top with air around it (s = stained clay, m = monster egg):
SSS
SMS
SSS

Breaking the monster eggs (after switching to survival) causes the silverfish to spawn inside the bottom layer of stained clay, which they quickly move out of. I'd have expected they'd spawn at the location of the broken egg, and therefore be trapped inside the clay structure.