Here is the explanation from Adobe I was looking for:
Using Alt + Delete to fill a layer on second Artboard makes that layer larger. Why?
The second to last response in that thread was very helpful to me:
Whats not clear to most users is that every layer in every artboard is really the same size as the overall image size. We trick Photoshop into treating artboards like separate docs but it is important to know that they aren't. This is why artboard files may seem disproportionately large, what appears to be many small layers can add up to a very large file. |
I read this four months ago so I'm not sure I did a great job with my earlier answer—I hope that the combination of both will fill in that you were missing about artboards in Photoshop, and explain the larger that expected file sizes associated with them.