Fat Santa has 4 bonus features. With Santa’s sleigh dropping off Mince Pie Wilds onto the reels randomly in the base game, the focus is on the Free Games feature where you’ll play with a 1x1 Santa Wild. When Santa eats enough mince pies, you’ll win extra free spins and fatten the Santa Wild up to 5x5 in size.
Santa’s Sleigh

Randomly on a base game spin, Santa’s sleigh will fly across the screen to trigger the Santa’s Sleigh feature. Santa will leave a trail of Mince Pie Wilds behind on random reel positions.
The Mince Pie Wild symbols lock in position with the reels respinning. Essentially, it makes it a lot easier to form winning combinations.
Free Games

Fat Santa, like its farm-themed counterpart Fat Rabbit, is all about the Free Games feature. It’s triggered when a Santa Wild symbol lands with 1 or more Mince Pie Wild symbols anywhere on the reels.
5 free spins will be awarded with a 1x1 Santa Wild symbol on the reels that moves around with every new free spin. On the reels, Mince Pie symbols will appear. The Santa Wild will race across the reels to eat them. The more he eats, the more the Santa Wild will grow.
With every Mince Pie symbol eaten, the meter to the side of the reels fills up. If you fill the meter, the Expanding Wild Santa feature comes into play. Every time you fill the meter, the Santa Wild increases in size. Additional free spins will also be won.
Importantly, the Santa Wild can reach up to 5x5 in size with up to 13 free spins to be played in total. Below is an image highlighting how many Mince Pie symbols you need in each of the levels to increase the Santa Wild size and win more free spins:

If you collect the maximum amount of Mince Pie symbols, the 5x5 Santa Wild will guarantee a win of 1,000 times your total bet on the last free spin. Theoretically, you can win up to 10,233 times your total bet in the Free Games feature as a whole.
Bonus Buy
Push Gaming gives non-UK players the chance to buy their way into the Free Games feature. Known as the Bonus Buy feature, it will cost you 80 times your total bet. The RTP rate also increases to 96.59% (for the highest 96.45% RTP version).