The problem I have found even with the two seats is that my issue is not butt room (although I have no doubt I'd spill over - but if I was traveling with my 2 year old, I wouldn't care) - it's with the leg room. You may find even on Southwest that even if you have the extra seat, your knees are straight in the back of the person in front of you and getting up to let the person by you (or going by the person) is extremely uncomfortable.
Which is why Spirit works out so well for people of size. Go figure, that one of the most customer unfriendly airlines is one of the most fat friendly-cost efficient airlines..
You can fly, for instance, from DFW-PDX in their first class upgrade seats for $18 each way! Vegas is $65 each way and the most I've seen. LAX is $50. FLL is $40 one way and $60 the other way. You get 38 inch pitch and 20 inch seats with big recliner like arms to keep you "contained" within the seat. At 6'5, 375 with most of my weight below my belly button (think pear instead of apple) - I can use one of their seatbelts without an extender. And if you get their Spirit credit card, and again, play their game, the frequent flyer program looks amazing - like a hidden gem. I feel like an employee - but I'm just thrilled that there's an airline, if you're willing to jump through a few hoops, makes it convenient for big people to fly (and cost effective).
Of course, the variables others have mentioned - where you are flying to/from matters since Spirit is not everywhere..