Chubby - I am a big man, well over 200 lb, and I feel very secure on a decent mountain bike with a good steel frame and good wheels, which is almost found on any name brand. There were millions of these made by different companies, find a nice one in your size for around a hundred. Then get it serviced and put on street tires that are around 2 inch width, these bikes make excellent city riders. You may want to change the handlebars too, try to find a bike that has a "riser" bar as opposed to a straight bar. Lastly, get yourself a quality seat ! I just got a new Brooks B67(about $100), which is for upright riding and has big coil springs in the back. There is a reason why Brooks seats have been around for over a hundred years, they are great and will last for several bikes that you may own, just swap it over.
I would stay away from skinny tire bikes for a while, you will be surprised at how well a "mountain bike" rides in the city.
Best of luck -