Tech details:
55AX - weighs in at a svelte 18.5 oz. and has the same foot print as the 46FX and 46AX siblings - hp 1.7 (hp/oz .092)
61FX - hits the scales as 23.6 oz with a bigger foot print that is shared with the 75AX and 91FX brethren - hp 1.9 (hp/oz .081)
75AX - em, 8 more oz. of nose weight over the 55AX at 26.5 oz - hp 2.4 (hp/oz .091)
91FX - 24.3 oz - hp @ 2.8 (hp/oz .115)

75AX gives you a bunch of hp at the cost of 8 oz of nose weight. A 91FX is lighter - go figure

for comparison
Saito 100 shares the foot print of the 61/75/91 platforms - wt. 20.9, hp 1.8, hp/oz .086
OS FS91 shares the same foot print as the 61/75/91 - wt. 24.2, hp 1.6, hp/oz .066

All of these numbers came from respective manufacturers.

Btw, the 61FX group and Saito 100 share the same size mount. I just mounted the Saito 100 on a Hog Bipe with a Hayes AL60 mount. The mount fits nicely. Allows upright, side or inverted using the same blind nut locations. If I get a wild hair (or hare) I can grab another AL60 and toss one of the OS 2-strokes (61/75/91), new throttle control & tail weight - just to see how it goes.