A better picture of the profile may help of both the new post and the existing base.

If this is outside, then plywood would not be a good option. I agree, we need better pictures

return and buy what fits?

Smaller poles or bigger bases would seem to be the two choices in this situation. Adding new concrete on to old doesn't typically work very well in a situation where you have anchor bolts under sideways stress. Could probably be done with enough rebar, but at that point it might be faster and easier to just pour new bases that are big enough. Or, even easier, get poles that fit the bases you have.