Not sure why people keep bringing up a Switch when the Steam Deck isn't even aimed targeting the same audience as them. The people who are interested in this are most likely people who are already invested in Steam and PC and not necessarily interested in consoles and vice versa. Yes, it is big though its not THAT big that you can't carry around with you and use comfortably. Though like the Switch, its probably more at home to be used as a portable device indoors rather than out.
Storage wise, it accepts Micro SD (think up to 1TB) cards for expansion and also read from external USB drives. And if you're more technically inclined, the internal memory is user replaceable with a little tinkering.
Cost is a big factor in the appeal for this device. Like try finding a PC/laptop that has similar specs that can play the latest games natively on the platform for $399. I bet you can't. The only other alternative devices in this class cost 3x more than the Deck take the; Aya Neo and GPD Win Max. This is literally an all in one PC in a handheld form factor, that cost about the same as a console. Has a full operational OS and is open platform. Can play hundreds of games if not thousands (on Proton) of games at launch. For value alone its and insane package.. but for some reason people can't see past ItS TOo BiG so FaILuRE.