If the wheels spin freely and there is no brake drag then you are feeling the weight of the entire bike. If you are used to a light road bike and suddenly try riding a bike with bigger tires, a rack and panniers that weighs probably twice as much, you will definitely notice it. You can't compare it to your MTB since you probably don't ride them on the same terrain and the MTB is probably still lighter than your commuter.
i have a Surly Cross Check set up as a rain/errand bike with 700-32 "City" tires at 80 psi, a rack, rack trunk and fenders that weighs over 35 pounds. Compared to my road bikes at under 20 pounds on 700-23 tires, it feels like a boat anchor.
Last edited by HillRider; 10-18-11 at 10:44 AM.