It's hard to believe that just last week (a mere 168 hours ago) they were hatching out of their little eggs and taking their first bite of a tomato leaf. Now they're seasoned tomato leaf-devouring pros who are consuming their own mass in leaves daily. They're growing up so quickly!
The caterpillar pictured above is in its third instar, meaning that it's molted twice since it hatched. It's near the end of its third instar, which you can tell by comparing the size of its head capsule to its body. Hardened portions of a caterpillar's body (its head capsule) can't expand in size within an instar; the caterpillars have to molt if they want to get a bigger head. However, soft portions of a caterpillar's body can grow in size, so as a caterpillar grows within an instar their head capsule gets proportionally smaller than their body. One method to determine the instar a caterpillar is in, without closely monitoring its development, is to measure the animal's head capsule width.
When it comes time to molt, the caterpillar doesn't bother with trying to expand its old head capsule. It grows an entirely new head capsule behind the old one, digests away all the material contained within the old head capsule, and then just tosses the old capsule aside.
This picture shows a M. sexta that is in the middle of its third molt (between the third and fourth instars). To see the molt in progress, compare this picture with the prior one in the post. The head capsule is more translucent than normal, due to the digestion of contents within. The head capsule is also starting to be pushed off the caterpillar's body (it's further forward and lower than normal) as the new capsule grows in behind it. About one of the only things that the caterpillar hasn't digested off the old capsule are its mandibles - they're the two little black spots that you can see at the bottom of the old head capsule.