Jamieson, Fausset and Brown on Galatians 6:2

2. If ye, legalists, must "bear burdens," then instead of legal burdens (Mt 23:4), "bear one another's burdens," literally, "weights." Distinguished by Bengel from "burden," Ga 6:4 (a different Greek word, "load"): "weights" exceed the strength of those under them; "burden" is proportioned to the strength.

so fulfil--or as other old manuscripts read, "so ye will fulfil," Greek, "fill up," "thoroughly fulfil."

the law of Christ--namely, "love" (Ga 5:14). Since ye desire "the law," then fulfil the law of Christ, which is not made up of various minute observances, but whose sole "burden" is "love" (Joh 13:34; 15:12); Ro 15:3 gives Christ as the example in the particular duty here.