I've been weighing my grounds as part of trying to up my game a bit. But I'm confused about brew ratios and shot volume/weight.

With (what seems to be) a typical brew ratio of 1:2 (dose weight to shot weight), and a dose of say 18g, I would expect a shot weight of 36g. However, I find that to get anywhere close to a shot volume of 2 oz. requires much more than 36g shot weight.

So I tried weighing just water (which I presume is at least no heavier than a shot, probably lighter since it has less solids?), and I find that 2 oz. of water weighs about 60g --- much bigger than what I'm expecting.

How do I reconcile this? Should I use volume in the cup as the guide? Weight?

Crema is the answer, it weighs a lot less than water.

yeah, always use weight. volume will change for all sorts of reasons, crema being the main one.