About Large object Storage

Hello,

I am using postgresql 9.3 in Windows. I take data backup using pg_dump. The
backup file size is 5 MB only. After that I insert 8 images using large
object.

The total size of 8 images are 24 MB. After insert large object only, I
take data backup using pg_dump. Now the backup file size becomes 246 MB. I
really shocked. so I check database size using the following query

SELECT pg_size_pretty(pg_database_size('testdb'));

The return value is 710 MB.

What is the problem? How DB size increased extremely?

Thanks for your Suggestions.