The UI on my phone was tiny compared to when I ran it on my computer. Also even though I haven't tried on other devices, I figured results might differ widely depending on their resolutions.

It's actually possible to make resolution independent UI. As explained here, select your Canvas element in the Hierarchy, then in the Inspector, set the Canvas Scaler's Scale Mode to Constant Physical Size.

I wonder why it's not the default.

There is more in-depth documentation on how to make UI for different resolutions and screen sizes at https://docs.unity3d.com/Packages/com.unity.ugui@1.0/manual/HOWTO-UIMultiResolution.html. But for my purposes, constant physical size is perfect.