At around 135lbs, I'm obviously constantly ragged on when I set fast laps at the local indoor tracks for being able to do it because of my weight advantage to most adults persons of the male gender. I can't say it's not justified.

But how much of a difference does weight make in indoor karting? First off, weight will generally make a much more significant difference in karting vs. cars because karts are many times lighter then even a small open wheel car like a Formula Ford. However, a recreational style kart is significantly heavier then a pure racing kart. A pure racing kart might weigh 180-200 lbs without the driver and while I don't know the exact weights of common rental karts, I'm guessing they might be over 50 lbs heavier. So, just considering the mass of an average sized adult relative to the weight of the vehicle, a weight difference in indoor karting will be significantly greater then in car racing and somewhat less then in traditional outdoor karting. This weight difference means that a lighter driver will be able to accelerate out of corners faster and generally archive faster speeds at the end of the straight, and thus faster lap times. The lighter driver will also have a huge advantage on a standing start.

However there is another factor that might not be as obvious. On a kart, more weight may actually mean more grip in the corners. This is particularly true on indoor tracks that have a very slick polished surface using rental karts with hard long lasting tires. The more slick the surface, the more of an advantage extra weight will be; on tracks with a conventional/grippy asphalt or concrete surface (indoor or outdoor), the effects of any extra grip may be negligible. So on a slick surface, is this extra grip enough to offset the disadvantage in power to weight ratio compared to a lighter drivers? From my experience, 99% of the time, no. Being lighter is still an advantage. However, how big of an advantage on a slick surface could depend on the specific kart and the track conditions.

Besides the track surface, the track configuration could also affect the size of the advantage of lighter drivers. From my experience the extra grip from extra weight is a more of an advantage in higher speed corners, so a slick surface on a high speed track would likely be the “fairest” for heavier drivers. A higher speed configuration also generally means less accelerating which also helps heavier drivers, even on grippy tracks where extra grip is insignificant. Naturally, most grippy tracks are higher speed so a heavier driver is not necessarily at more of a disadvantage at a high speed grippy track vs. a lower speed slick track.

Finally, a heavier driver also has more weight over the rear axle of the kart, which is where the only brake on most karts (and all indoor karts I've seen) is. To me, in theory this means a heavier driver will be able to brake better; however considering how marginally brakes are used in indoor karting I'm not sure if this effect is significant.

I have limited experience in traditional higher performance outdoor karting, but my feeling is these advantages for heavier drivers might not carry over to this discipline due to the lighter kart weight, much stickier tires and perhaps other factors, or at least be a much smaller effect.

In conclusion, lighter is faster in indoor karting, but maybe not by as much as you think.