I've noticed in conversations that a lot developers really do take page size for granted. Their rationale is usually that their target audience uses broadband so there's no need to worry about it. I call that naive thinking. You still need to be cognizant of what you're building and how its coming across the wire. Large pages can still be excruciatingly slow, even on broadband, so you can't take that for granted.
Well, Yahoo! just made it way easier to see what's slowing down your pages. The Yahoo! YSlow Firefox plugin “analyzes web pages and tells you why they're slow based on the rules for high performance web sites“. Considering that Alexa ranks Yahoo! as the top visited site on the Internet, I would say they know what they're talking about.
What I really like about this plugin is that it integrates directly with the Firebug console so you don't have some extra widget sitting in FF and it also provides a summary of the target areas that you should address to improve site performance. This summary is in the form of links which take you to an explanation of why a specific area is important. I'm really digging this.
If you're really serious about having a responsive site, you should consider adding Yahoo! YSlow to your toolbox.
Update: Stoyan Stefanov has done an excellent review of YSlow here.
