The performance of aircraft is greatly affected by weight. A heavier aircraft requires a larger engine and wings. Also, the weight limit tends to restrict the distance the aircraft can fly because each gallon of fuel weighs about 6 lbs., deducted from the useful load the airplane can carry.
LSAs are limited to a maximum certified gross weight of 1,320 lbs. or 599 kg. Gross weight means that everything is included: you, your passenger, luggage, fuel, lunch, etc. It’s also called the maximum takeoff weight. Ultralights are limited by their number of seats (one in most models, two in trainers) and empty weight (without pilot or gas). Gross weight makes more sense because it gives the designers a specific takeoff weight that they can work with rather than an approximation depending on how heavy the ultralight pilot is. The figure of 599 kg is similar to that used by the “ultralight” class of aircraft in Europe.
"Man cannot discover new oceans unless he has the courage to lose sight of the shore."
-- Andre Gide