Linux Compressing PNGs on the command line

I have 103 images I have processed with ImageMagick, but for some reason the files are humungous.

But when I open them in GIMP and do nothing but save it, it reduces the size to a quarter of what it was. Is this a bug in IM, or is there some flag I have overlooked?

 

Give it the -quality 100 and -depth 8 switches
the -quality is the compression, bigger == better but slower
and -depth is well read the manual, i normally use 8