Slot library size is the easiest number to compare between casinos, but it is a weak signal on its own — what matters more is which studios are behind the games. Fresh Casino, for example, works with Pragmatic Play, Evolution, NetEnt, Playtech, Play'n GO, BGaming, Wazdan, Amatic and Nolimit City, giving it both mainstream hits and higher-volatility niche titles from smaller studios that some larger casinos skip entirely.
Progressive jackpot slots pool a portion of every wager across a network of casinos, which is why the same jackpot title can show a different prize total on different sites — check the live jackpot value on the casino itself rather than a marketing page, since it changes constantly. Fixed-jackpot and "Hold & Win" style slots, by contrast, pay a set maximum regardless of network size, which some players find more predictable.
Volatility is worth understanding before you pick a session budget: low-volatility slots pay smaller wins more often and suit a longer session on a fixed budget, while high-volatility titles (common in Megaways and bonus-buy formats) can go long stretches without a payout before a larger hit. Most casinos list volatility in the game info panel — it is worth checking before you commit a bankroll to an unfamiliar title.
Finally, slot RTP (return to player) is set by the game studio, not the casino, so the same slot pays out at the same long-run rate wherever you play it. What does vary by casino is whether a promotion's wagering requirement fully counts slots at 100% — some casinos weight certain high-RTP or bonus-buy slots lower specifically to prevent bonus abuse, so check the excluded-games list in the bonus terms.