File size of thumbnails increased

I´m wondering about the automatically created thumbnail files in IMCE ...
I set up a profile where defined that thumbnails can be created in the size 650x650 .... this is working fine but if I lokk to the file size the created thumbnail is much bigger than the original file ....

Example:
original picture was uploaded: file size 50,48 kB
created thumbnail in 650x650: file size 126,71 kB

Could I change this behavior?