It's just basic extrapolation. Growth in terms of capacity/$ for solid-state memory (whether RAM, NAND, etc) tends to be linear w.r.t. the base 2 logarithm of the capacity. Or, to put it more simply, it tends to double every X amount of time, for some X. ASH and RE:R are the first uses of the largest capacities used for both DS and 3DS cards respectively, so seem a reasonable basis to judge the largest capacity which would be financially feasible at any given point in time. Put in graph form, you get this:
Obviously one shouldn't infer too much from two data points, so I'm actually being rather conservative about my assumptions; a simple reading of the trend line would actually suggest that 256GB cards would be feasible in 2017. I'm also ignoring the fact that both of those games included on-card flash memory for save games, which I expect Switch game cards to do without.
In any case, regardless of the precise maximum capacity which will be available, over a span of just over four years from the DS to the 3DS maximum game card sizes increased by 32 times. There's no reason to believe that progress somehow ground to a halt in the five years since then.