I've not read up n how to adjust the belt on 201s, but the 221 featherweights have advise on the FeatherweightShop website. Scary but it works - put on the belt, leave the adjustment (of the motor position) loose, and run the machine with your foot down. Now carefully move the motor about in it's range and find where the machine is running the fastest - that's it's optimum position. Tighten it up to stay there.
There should not be too much strain on the belt - sounds like it's too small?
Do you have all the manuals for the machine (sorry if I've asked that before)?