You're misunderstanding the seat upgrades.

The small, medium and large upgrade is based on the 211,000 seat capacity of your world arena.

The large upgrade adds 49,000 seats to your 211,000 stadium for a total of 260,000 seats. It does not add 49,000 seats to your world arena with a medium seat upgrade already built.

Each seat upgrade makes the next seat upgrade a little bit cheaper.

After building your world arena, the price of the 3 upgrades were: 137M, 231M and 341M.

Now, the large upgrade is only 110M since you have already built the first two upgrades and taken your stadium to 235.500 seats.

The last upgrade adds 24,500 seats for 110M to bring your stadium to the final capacity of 260,000

If you go back to your first sentence, you will see an error in your math. Your world arena starts with 211,000 seats. If you added 12,250 and 24,500 seats you would have 247,750 seats. What actually happened was the small seat upgrade added 12,250 seats to take you to 223,250 seats and the medium upgrade added another 12,250 seats to take you to your current 235,500.

I'm going to guess you have only done stadium upgrades while playing the game. Take a look at my seat guide in my sig file to get a better understanding of seat upgrades.