HCA: Hermit Crab Association

I've had my hermits for exactly a month and a day now. I'm under the impression that the bigger one is currently molting because they changed their shell, dug a hole and chilled for a couple days, then came out for a couple more and has gone back under for about 2weeks now. The little one is literally spilling out of its shell, I feel it's gotten too big for it but the critter just won't switch even though I've boiled in salt water and have a good 15-20 options for my guy... whats up with that?

Sometimes they prefer a tighter fitting shell, like skinny jeans. Or sometimes they might sacrifice space for weight - a tighter fit but the shell is lighter. What type of shell is he in now?