Album art in iTunes? The increase in file size is huge...why?

I just noticed this right now. Basically I'm in the process of converting my entire collection to iTunes file structure/format and am reembedding artwork. I've been using google and I can find a file that's approximately 125 kb's (600 x 600) and DRAG the file from the safari window into either the album art box in the left hand corner and/or the get info box and the size of the files will increase significantly.

For instance, I grabbed a 135 kb album artwork for Adele 21 and it jacked up the size of the album by 10 mb's. Alternatively, if I save that file directly to my computer and drag it, as a file, into that same box, the size of the album only increases proportionate to the size of the file so 135 kb's x 16 songs (approximately 2 mb's).

Is there a reason fro the difference? Does this have to do with iTunes converting the file to png if dragged from the safari window.

I was purposely avoiding higher quality artwork cause the file sizes were increasing so significantly. I was more worried downstream when I dragged these albums to my iPhone.

I just want to make sure I'm not missing something. If this is the route that I have to go then that's fine with me. I'll proceed with saving each individual file and going about it that way.