If you need to enlarge a round tenon, here's a trick that you can use.

Use a hand plane to get some shavings from a piece of wood that is the same species as the tenon. The longer the shavings, the better. Apply a coat of wood glue around the tenon, and then wrap one of the wood shavings around the tenon. You can do this for as many layers as are needed to get the desired size. You want a tenon that is slightly too big for the mortise it will be going into.

Once all of the shavings are applied, wrap rubber bands around them to hold them in place until the glue dries.

Once the glue is dried, you can then use a strip of sand paper to bring the tenon down to the correct size.