Toddler diapers are so thick when wet because the diapers contain a gel caused SAP or super absorbent polymer which absorbs the pee.
As the gel in the diaper absorbs the pee the gel swells up and that causes the toddler diaper to swell up which then makes the diaper thick between the toddlers legs and then you can see the diaper bulge in the front of a toddler's pants and the back of the toddlers pants.
However when the diaper is dry the toddlers diaper is thin and you cannot really tell the toddler has a diaper on.
However when disposable diapers first came out they used to be thick and you could hear the diaper as well as see the diaper even when dry and the toddler would waddle a bit in the the thick plastic backed diaper.
Toddler diapers are not really all that bulky now although toddler diapers used to be really bulky when they first came out.
As for today's disposable diapers they are actually pretty thin but the toddler diapers do become bulky between the legs when they swell up.
When the toddler pees in the diaper it starts absorbing as the gel in the diaper swells to lock in the wetness.
So if you notice a toddlers diaper getting bulkier and swelling then it's a sign that they have peed in the diaper.
The more pee the diaper has the bigger it will get until it cannot hold anymore pee.
This then makes it hard for the toddler to walk as it creates bulk between there legs like they are wearing a balloon between there legs.
Then when they are changed into a fresh diaper they can walk better.
When I was a baby the diapers me and my brothers wore were actually thick and not as thin as today's diapers so when we walked we would be waddling and you could see the diaper bulge in our pants and clothes.
Today you cannot see the diaper bulge in the pants until the diaper is wet.
Diapers are made thinner now to make it easier for the toddlers to walk around and to make them more discrete for the toddler as well.