I'm 13 and-a-lot stone and I've won my last two races at Milton Keynes Daytona.
Cornering tips:
If there's a long straight after the corner, brake slightly earlier and get the kart settled so you can plant the throttle on or before the apex - taking a late apex into a hairpin can also help with this. It's most satisfying out-dragging a 9 stone scrote when you get a 5-7mph head start
Only brake where you need to - seems obvious but there's probably a lot of occasions where you need only to lift rather than brake. Most time this is on long sweeing corners.
Be prepared to sacrifice 1 corner exit for another - If you have a quick left-right section, brake a little more than you'd think to keep the kart tight to the inside of the left. You can then plant the throttle through the rest of the right turn for a slingshot out of the corner.
Use the run-offs - on outdoor tracks they sometimes get a bit narked if you keep doing it, so try it first and see if you get warned. Depending on the track you're on theres some curbs which you can lean on, some will slow you down. Only one way of finding out which is which.
Learn where to position your kart - this only really comes with racing experience, but knowing how to confuse someone in front of you, and how to block someone behind you are the most important things to learn. If you've got Forza, crank it up for some practise - I find the B class cars are quite a similar experience, lots of grip and easy to throw around.
MOST IMPORTANTLY - If you're being lapped, get the fark out of the way!