I'm noticing that unlike in photoshop, pressing down harder with my pen doesn't make the actual pen have a larger swipe, instead changing the opacity of the line. Is there any way to remedy this?

That depends on the brush preset's settings -- i.e., which preset are you using? You can easily change, though, in brush preset editor. Just connect the pressure sensore to the size parameter. See https://docs.krita.org/Parameters#Size