Q:

Mobile page seems really large. Am I loading the mobile page, or the normal page?

When visiting stackoverflow on my mobile phone, it seems to be downloading a lot of data. For the home page, it shows somewhere over 500 K near completion, although I can't tell the exact size since the progress bar disappears on my browser.  I'm using a Nokia C5, with a user agent of:
Mozilla/5.0 (SymbianOS/9.4; Series50/5.0 NokiaC5-04.1/10.1.031;Profile/MIDP-2.1 Configuraion/CLDC-1.1) AppleWebKit/525 (KHTML like Gechk) Version/3.0 BrowserNG/7.2.6.9.3gpp-gba

My Accept Encoding header is 
gzip, deflate, x-gzip,identity; q=0.9

Why is it downloading so much data?  Shouldn't the mobile site be smaller to account for people without unlimited data plans?

A:

I'm not sure where you're getting 500K from.
The mobile home page on a cold cache is somewhere around 17KB compressed with gzip.
The desktop home page is something like 32KB compressed with gzip.
These figures are coming from Fiddler.
Perhaps you're including images or ads with that number?  Even that doesn't make much sense, as all the images in the mobile skin put together are only ~4K (there are no ads); and you should be getting cache hits on those constantly.  Even the desktop images (including all tag sponsorships, the offline, 404, error pages, all the fancy wmd controls, about, and faq images) only total 401K, and they're never all present on one page anyway.  Honestly, that indicator sounds like it's garbage (perhaps counting uncompressed size and cache hits?).
For your user-agent we do not serve the mobile theme by default.  We don't test with that device, and the Stack Exchange desktop themes are much more forgiving of older or less compliant browsers*.  You can opt into the mobile theme (assuming you haven't already) by using the "mobile" link in the footer of every page, and opt back out with a similar "full site" link in the footer of every mobile page.
*Thus making the desktop theme a safer default for unknown devices.

