I just did what seems to be more damage when using Double Kick than Low Sweep to the same Pokémon, was it a high roll or...?
No. Double kick get STAB for both hits, each of which have 30 base power. So after STAB double kick would have 45 BP for both hits or 90 base power. Low sweep is 60 BP, after STAB it becomes 90.
So in conclusion, Double kick has the same BP as Low sweep. You probably got a high roll on the mon you were using double kick on.