pg_dump's results have quite different size

Hi, I have postgresql 9.4.10, CentOS 6.5, 64 bit. I got the source codes,
built and gmake install.

I pg_dump the database 3 times everyday. The weird thing is that the sizes
of the dump files are sometimes quite different.

For example, the file dumped in 7am is 1.5G and the file dumped in 11am is
0.91G -- about 60% gap! But no massive deletion happened. Only some
insertion/updating (less than 10K) in between 2 dump operations. Un-logical
to have different sizes.

Note that in about 70% cases, file sizes are similar to each other and
keeps increasing a little bit, which is very logical. But sometimes I got
weird things as described above.

Is that a bug? Or something else?

Regards,
Kaijiang