Text _segment_ size. That's executable instructions; presumably it makes a bit more sense that Google would have a binary with 300MB worth of instructions laying around.
I repro here some data, there is a bit more in the paper:
Clang 1.9k Files, 217MB (-g0), 3554MB (-g) Chromium 17.8k Files 706MB (-g0), 7544MB (-g) Ad Delivery 13.8k Files 1073MB (-g0), 7469MB (-g)
(this is the number of object file that are participating in the link).
Ah, that would probably be it.