Much of your hermit crab is hidden in his shell, so it's probably not obvious Hermie's growing. Hermit crabs have rigid exoskeletons that don't expand with their growth. When your crab becomes too big for his skin, he needs to molt and shed his exoskeleton. Molting is a stressful, potentially dangerous, necessity for a crab.
In the pre-molt stage, known as proecdysis, you may notice Hermie's developed a big appetite, eating and drinking more than usual. Hermit crabs don't eat while they're actively shedding their exoskeleton, so they prepare by storing food, water and salts to see them through the process. If you see a grayish black bubble on Hermie's abdomen, don't be alarmed; the sac is the food and water reserves he'll rely on during the molt. The excess water he's taking in during his pre-molt stage also helps him break open his exoskeleton.
Relying on Reserves
Most hermit crabs go underground to molt, which is much safer than a surface molt. While Hermie's under his substrate, he won't be eating or drinking during his actual molt, also called ecdysis. Don't worry that he'll starve during this period; he's well-prepared with the nutritional stores he built up during the pre-molt stage. As well, this is the shortest of the molting phases. Hermie won't eat or drink normally until he is finished with his molting process and emerges from beneath the substrate of his crabitat.
Exoskeleton for Dinner
Though Hermie isn't dining on his regular diet while he's buried, after he's molted he will eat. The post-molt phase, known as metecydsis, is when he begins to harden up and regains his ability to move. It may take a day -- or several days -- but during this time he'll eat his shed exoskeleton, which is an important source of calcium, salt and other minerals which help harden his new exoskeleton.
Rest and Growth
When Hermie's exoskeleton has firmed up and he's ready, he'll come out of hiding to join the rest of his pals in the crabitat. Anecdysis is the intermolt period, when his new exoskeleton bulks up and is a period of rest and growth for a crab. It's the longest phase, and as Hermie grows, it will become longer and he won't need to molt as frequently. Though vital for a crab's development, molting is extremely stressful for a hermit crab, so if Hermie's going through a molt, it's best to leave him be. Besides an increase in appetite and his sac of stored nutrients on his abdomen, you may notice more digging behavior, lethargy, cloudy eyes and a chalky cast to his exoskeleton. If it's molting time, make sure he's safe from other crabs and let nature take its course.