This is normal. Chickens to not have teeth so all of the food they swallow gathers in their crop (which is what you are feeling). The crop is like a balloon that fills with food that is waiting to be broken down and digested.
 
Chickens eat little rock and grit that are collected in their gizzard. The food will pass from the crop to the gizzard where it will be ground down by these little rocks using the strong gizzard muscle. This process takes about 4 hours so as the chicken eats the crop will fill.
 
Check your birds first thing in the morning. After they have been sleeping all night, their crops should be empty.
 
It is possible for chickens that eat a lot of long grass for things to get tangled in there and block the crop. This is called impacted or sour crop. If this happens the chickens will become sick - their crop will not be able to empty and it will keep getting bigger and bigger.
 
What you are describing however sounds like normal crop function.