A bit of background: I decided to try a tiny backpack as a purse and loved it so much that I wanted to try making my own. I've only made a few zippered bags here and there, so it was a big undertaking. But somehow, it went pretty well.
That was October. I've made fourteen more bags since then (but no more backpacks yet) including the Ethel tote by Swoon, which is a free pattern. You can find it here: https://swoonpatterns.com/shop/ethel-tote-2/ I decided to play around with it and make it more purse-sized, and here we are, shrinking patterns and seeing what happens.
The issue with shrinking a pattern is the seam allowances. If you shrink a pattern but don't change the seam allowances, there will be surprises as you sew it up. So far, I've only encountered one problem and it was easy enough to fix -- the gusset ended up being 1/2" shorter than the two main pieces that make up the front and back, so I just trimmed it down.
That's really the only thing that went wrong. Well, that and it's pretty darn small. I think maybe something like 85% would be just right. Also, it has a ton of structure because of the foam. It stands up on its own because of this, but it also naturally hangs open. I didn't plan a closure for it. It might not be too late for a magnetic snap closure, but I haven't played around with those yet.