A gaming device comprises a display, a processor, and a memory storing (i) a plurality of symbol position states, each defining a number of active symbol positions in each of a plurality of columns of symbol positions. Also stored in memory is data defining a plurality of sets of reel strips including a first set of reels strips, wherein each reel strip of the first set has a first stack length defining a number of consecutive reel strip positions containing the same symbol, and a second set of reel strips, wherein each reel strip of the second set of reel strips has a second stack length shorter than the first stack length. In at least a first game instance, the processor selects symbols from the first set of reel strips for display in the plurality of columns of symbol positions of the first symbol state. Responsive to a trigger condition being met, the processor updates the current symbol position state to a second symbol position state comprising at least one additional active symbol position relative to the first symbol position state. Subsequent symbol position states will have progressively shorter stacks in one or more of the corresponding sets of reel strips.