Why thumbnail png file is bigger after original png

I use png format picture file sometimes. It is OK when I upload the file into MovableType platform. I choose make thumbnail to 400px width from about 700px.
Perl ImageMagick engine will create thumbnail file. The blog software will generate html code and put into editor body.
Everything looks find. But the file size of thumbnail png is bigger than original png file.
Original png file: 700X505 49KB
Thumbnail png file: 400X288 78KB
Why is it bigger?

I opened these two files by Adobe photoshop.
Check picture Image Mode.
Original picture: Indexed Color – 8Bits
Thumbnail picture: RGB Color – 8Bits
I do not know why ImageMagick change Image mode when generating thumbnail.
It is Perl ImageMagick on Linux. Actually on Dreamhost.
Is there anyway to modify directive of mt-config.cgi?
When use jpg format picture, the thumbnail file size is OK.
Say 50KB for original file, 21KB for thumbnail.