Sometimes, but not always! Like, it might be a problem if you had six Charizards or something, but yours will grow up to be all sorts of different sizes. Some Pokemon don't even evolve, either because they can't or cause they don't want to, so it depends on a lot of things!

[Jimmy explains this while Ashley pets Lucy and Leader, with the Manectric appearing more receptive to the pettings than the Houndoom. At least, unless Ashley notices the way the Houndoom's spiked tail has begun to wag veeery slowly. Don't make a big deal out of it okay, gosh.]