For an adult yes. My boys can't even fit a 3DS in their jeans pocket.
Battery life? Meh, I have over half a dozen power banks in my house from 2k mA up to a couple big 20k mA. Watch the sales and you can get decent quality ones for fairly cheap. I gave out 6 KAMASHI 10K units I picked up on Amazon for $11 to various family members this Christmas.
I hardly go anywhere without one of my various day packs. I might need to get a different powerbank for the Switch depending on the whole 5-15V thing, but I am confident I can get a 15k to 20k unit for about $25 or less that will provide and additional 9 hours or so of runtime on the go without access to AC power. 12 hours of total runtime is more than I could possibly use in a 24 hour period.
Any larger battery in that thing will just make it heavier and more tiring to use.