Ah Flounder, there's a trick to that, and I know the answer! Not everything comes in "Full Size"!
In your Inventory, you will see that some of your pumpkins will say "Full Size" but others might say "Size 10", "Size 7" or "Size 4".
To find out which pumpkins you can make bigger, you will need to visit the Swap Shop!
The Swap Shop is located in the Mall. It's the 4th item on the drop down list for the Mall. You will see that the Swap Shop has another drop down list, and "2016 Pumpkins" is one of the things on that list.
When you select Swap 2016 Pumpkins, the Swap Shop will show you in red which pumpkins you have enough of to make into a bigger size...
Happy upsizing Flounder!