It's the same thrust, different amps
A 24V motor producing 80 lb of thrust isn't more powerful than a hypothetical 12V motor at the same thrust — it's pushing the same wattage through half the current. That matters because amps are what determine wire gauge, connector size, and how much voltage you lose over a long run from the battery to the bow.
Where the breakpoints actually fall
Most manufacturers cluster their lineups the same way: motors under roughly 55 lb of thrust stay 12V, the 55-112 lb range runs 24V, and anything pushing past 112 lb steps up to 36V. Going to a higher voltage at the same thrust class generally isn't necessary — it mainly matters once thrust climbs high enough that 24V amp draw would require unreasonably thick wire.
Cost and weight tradeoffs
Higher voltage systems need more batteries wired in series, which means more weight and more up-front battery cost — lithium especially. The payoff is lower amp draw for the same power, meaning thinner wire, less voltage drop, and often better efficiency at the top of the throttle range.
When to step up a class
If you're consistently running your motor at or near max thrust — heavy current, big wind, a loaded-down boat — a higher voltage class for your thrust range will draw less current and run cooler than pushing a lower-voltage motor to its limit. Use the trolling motor calculator to size your thrust first, then pick voltage off that number rather than the other way around.