One of the ninebarks (Physocarpus opulifolius)?

ok, so you don't think this is a weed. do you think someone probably planted it on purpose?

what should I do with it in terms of care? I don't want it to get very big. should I prune it down heavily this fall?

It's a weed if you don't want it there I would assume someone planted it there since it's purple-leaves.

Depending on cultivar they can get big. I have a couple that have done that and I cut them back severely in spring last year and they were fine.