From a gearing point of view it is the ratio, 'axle sprocket
teeth/engine sprocket teeth ‘which is important.
In your example 78/12 is a ratio of 6.50 -1…84/11 is a ratio of 7.64-1. Vastly different.
To keep the same or similar ratio with a smaller engine sprocket you need less teeth on the axle sprocket as well so to keep the 6.50-1 ratio needs 6.50x11=71 teeth at the axle.
Although 6t axle sprocket to 1t engine sprocket is a valid equivalence for a Rotax or similar , it varies with the gear ratio.
On my engine of choice ( Briggs v twin) gear ratio around 3.0-1. equivalence 3 to1.
At the other end of the scale a screamin’ 100cc might be runni
ng 10-1gearing on a tight track, equivalence 10 t axle sprocket= 1t engine sprocket.