Reward Pool Amount
By default, the DEC reward pool is set to 1M DEC every day. However, the reward pool is inflated based on the market price of DEC. Whenever DEC is over the soft-peg price of $0.001, the reward pool gets inflated based on the increase in price. For example, if the market price is $0.005, the reward pool will be increased from 1M to 5M DEC.
At any given time, you can view the total amount of DEC in the reward pool here.
Reward Shares Calculation
The DEC reward pool fills up a little with every block, meaning every 3 seconds when a block processes, a little bit more DEC is added to the pool. Then, whenever a ranked battle is completed, a value called "reward shares" is calculated for the winner. The reward shares value is the player's rating cubed and then adjusted for the various bonuses like if they used alpha/gold cards, win streak, guild bonus, energy capture rate (ECR), etc...
The reward shares are used to calculate how much of the reward pool a player receives. Players will receive their reward shares divided by the total reward shares for all ranked battles over a 24-hour period. So, for an overly simple example, if there were 10,000 total reward shares for all battles over the last 24 hours, and a player's reward shares for a battle is 1,000, then that player would receive 10% of whatever DEC is in the reward pool at that time.
At the beginning of a new season, every player's rating is reset, so their reward shares are reset as well. As a result, every player's reward share is compared to the total reward shares over the last 24 hours which is mostly comprised of battles from the end of the previous season when the player's ratings were much higher. This is why DEC rewards are a lot lower at the beginning of a new season, but then significantly higher near the start of the second day of the season.