Why does 'svnadmin dump' takes up so much space?

My repository now is totally only 70M, has 3000+ revisions, Now I want to
dump r2647:r2871, So I use such command 'svnadmin dump E:\myrepository -r
2647:2871 > f:\my1.dump', wait a long time, after it finishes, I check file
f:\my1.dump, it's already 500M.
Why does it takes up so much space(more than original repository)?

Question2: actually r2647:r2871 is the begining and end of a project in the
repository, but not sequent(Sorry for my poor english, I mean like r2650
r2651... is not revisions of this project), I just want to dump the data of
this project, and integrate the dump to another repository, Can I achieve
that, and How?
Thanks in advance!

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