Let's talk about AMMs (automated market makers):
The more of a token is swapped out of a liquidity pool, the less of it remains and the higher its price becomes, so large swaps result in greater slippage.
AMM quotes are determined solely by the ratio of the two tokens in the liquidity pool. When the pool's price differs from the price in other markets, arbitrage trades bring it back toward the external market price.
$UNI 7.134(+2.88%)