Publish package size doubles up


I am using REFramework and developed automation project. Every time, I make changes to my project, I publish nupackage, I noticed that fielsize grows to double. Is it because it is keeping older version also in package? If yes, is it possible to not keep the older version?
Even I correct spelling mistake in email subject and publish, the nupackage size is doubled up. I feel something is wrong or I am not aware of.

Pls help.