mzlamal may be able to add more, but:

SunOS hg 5.11 snv_62 i86pc i386 i86pc
Mercurial Distributed SCM (version 0.9.5)
Apache (some version), serving HTTPS

I don't know of any particular correlation to the amount of data pushed, though
that would certainly be a plausible place to look. I have heard people say that
they had been pushing fine for some weeks and one day the push failed with this
message, then a while later it started working again. I can't imagine they were
pushing _less_ when it started working again; if anything they would be pushing
slightly more (another merge changeset).