By default, in the Mac finder you can see the size of individual files but not folders (unless you click on “get info” for each one.
However it’s easy to set the option to see the folder size. Just go Finder, View, Show View Options, and select the “calculate all sizes” box.
Bear in mind that the reason this is deselected by default is that performance may become slightly degraded when it’s switched on. Personally I see no difference but bear it in mind.