Freaky! Image sizes increase for no reason?

Classic end-of-my-rope problem solving post. I hope the answer is easy, but I’ll give as much detail as I possibly can….

The goal:

I have two blogs — http://www.third-beat.com and the new http://www.third-beat.ca. I want them to match in style, and look completely the same except for the color scheme.

The problem:

http://www.third-beat.com is… bloated? If you open both sites up in separate windows (in Firefox only) and toggle back and forth, you’ll see the problem. I first noticed that the header was stretched out and had lost its clean edges. But it’s actually ALL the images that have been increased in size: the Google ad, the grey diagonal line thingy at the top, the images that create the main column and the sidebar… *everything* is stretched out. Even the images that are embedded into a post. In this post, for instance, the photo is showing up at approx 556px wide.
In reality, this 500px image is exactly what was embedded and it was even given width parameters of 500px.

What I did:

I toyed with the theme for http://www.third-beat.ca until I got it looking perfect. I downloaded the whole folder of files off my ftp. I edited the images to change the color and edited the hex codes in style.css, and then uploaded it to my http://www.third-beat.com. that’s ALL I’ve done to the theme. I’ve touched nothing else since finishing editing the .ca version.

How I’ve tried to fix it:

I’ve installed the latest version of WP. I disabled all of my plugins. I got rid of all the sidebar widgets. And – here’s the kicker – I viewed it in IE7 and it looks PERFECTLY FINE. I toggle between the two sites and they look identical.

I don’t posses killer css skillz, but what’s killing me is the LOGIC of this. It makes no sense to me. Does it make sense to you?