The Small Frame 20L Rucker is 1.5" shorter than the Standard 20L Rucker - ideal if you’re a smaller person or have a short torso.
The 25L Rucker is 2" taller than the Standard 20L Rucker - ideal if you're taller and plan to use the Padded Hip Belt often (it's called hip carry, not belly carry) or want to ruck extra heavy with a 45LB Ruck Plate® + 20LB Ruck Plate® for up to 65 lbs.
There’s no hard and fast rule to rucksack sizing, but the belt test is our clearest guideline. You want the bottom of the rucksack to sit above your belt line. If the bottom isn’t above the belt, then you’ll introduce the potential for friction along your belt line. We’ve found that most people under 5'6" pass the belt test with a Small Frame Rucker.