My GZ 's top speed seems low

[paulisnofun]

After a short amount of time practice riding in my neighborhood, I decided to actually taker her (2002 GZ250. 4500 miles) in to the real world. Yesterday was the second time I rode to work, and noticed that the bike doesn't seem to have the (for lack of a better word) balls that I thought it would. Reading on the forums, I saw that people had a top spead of around 70 miles per hour, and I am not close to that. I have about a half an our comute to work, no highway, just plain old suburban roads (mostly flat). The max speed limit on my route is 45, so I am not looking to go 75, but feel like maxing out at around 50 isn't right. That is 50 in fifth gear, with the throttle totally open.
Now, I am a bigger dude. I am six feet tall and weigh about 245 or so without my helmet and laptop bag. Would the fact that I am about as many pounds as my bike has CCs lower my top spead by that much? If not, where would be a good place to start looking to see where the lack of power comes from? I believe the person I bought the bike from said that a tune up had been done shortly before he sold it to me. Any help/advice would be great.
Thanks.