Arbitary file size limit in twophase.c

There's an apparently arbitary limit of 10,000,000 bytes in twophase.c
on the size of a two phase commit file. I can't see why this limit
exists.

I hit this limit by creating a prepared transaction which included
dropping a schema with about 25,000 objects in it and then trying to
commit it. The result is that ReadTwoPhaseFile() returns NULL and
FinishPreparedTransaction() says it detected a corrupted file.

Thoughts?

Thanks,

Gavin