There are a couple of points here:

1. As originally written, the change crashed test/format because pages were being forcibly evicted from under verify (see http://mjc.homeunix.org:8180/job/wiredtiger-test-format-coverage/773/console for details).

2. The page size check is necessarily imprecise, and it is not that difficult to create a page where the in-memory representation is 20x the on-disk size, even for a clean page – for example, by expanding lots of keys. We wouldn't want to forcibly evict such a page every time it is read.

Stepping back, the intent here of the page size check is to make sure that busy pages don't grow without bound. Read-only pages are not growing, so they can be evicted by LRU regardless of how big they are.