Lots of good tips from Mark, Tricky, Kato and Decrepit.

I am 73 'ish Kg/ 175cm, and almost always feel at a big disadvantage beside the larger sailors.
My strategy is:
1. To play the efficiency game. Use the smallest board, sail and fin I can to be fully powered without extra weight and drag.
2. Learn to be in the right place at the right time on the right gear to make the most of what you got.
3. Finesse your board, sheeting angle. Especially course angle if possible.
Regarding point 1: There is only so much power one can hold for your size and weight. Not having more sail than that reduces drag and weight (rig weight also = drag). Better to use a smaller but efficient sail with a lower centre of effort and perhaps a deeper profile to get max power per sq/m. That often heard advice to go as big as humanly possible is just plain wrong.

, but sometimes one has to have a larger than optimal sail to get through the lulls if it is a gusty or holey wind, and on the right size you
will feel overpowered reaching.
Try to get a setup and stance that gives you maximum leaverage over your sail. Think longer harness lines, wider boom and closer foot straps.
The same applies to fins. Use the fin size that has enough lift to get you control at medium to max speeds, This is not going to feel that good at lower speeds. Use an
efficient fin whenever possible. Again, not always possible where there is thick weed, but even in those cases, don't go for 'safety' and convenience' too much. Bite your lip and put up with a bit of inconvenience, or harder upwind sailing to get better speeds on the runs.
Regarding point 2: Watch the wind on the water to see the gusts coming. Watch the clouds and other signs for timing. Learn the rythm of the wind, gusts and squalls, at your spot in various conditions.
Regarding Point 3: Don't muscle it with dead feet and hands. Don't oversheet. Learn to 'feel' the ideal angle off the wind and ideal sheeting angle. Experiment with you rig settings. If other fast sailors are doing something and you try it but it does not feel as good, maybe it is just because it is unfamiliar. Persevere and be objective. Change stuff if it is not working. Change stuff even if it does feel good to learn the difference and find out if it really is best.
In winds under around 28 knots I have often found that it is faster to stay slightly less broad and keep some pressure on the fin. It is easy to go too broad and outrun your apparent wind. In stronger winds, usually borader is better. Don't be rigid about your course angle. Try to react to the gusts and anticipate.
The point Decrepit makes about footstrap positions and mast foot positions is very important. Most slalom and speed boards were designed for big guys. Find the ones that work for smaller people and pay particular attention to footstep positioning and spacing. Closer together feet gets your mass higher and gives you more leverage on the sail when on a broad reach. It's a tradeoff for less leverage on the board when square reaching or upwind though. I used to set up with closer on the speed side and wider apart for the upwind side. Now, I usually just go narrower all the time, then I can do runs on either tack. Boards with a little less flat in the tail may work better for a lightweight to 'fly' the board better. Remember that slalom boards are almost always set up for large, tall people, and mainly for reaching.
That said, I was recently reminded at LG that to get max speeds off the wind in strong winds 30-40k), I WILL feel quite uncomfortable and overpowered sailing upwind in the squalls. Sandy Point is a spoiler in that respect, because when it is blowing 'dogs off chains', there is no need to sail back upwind, you can walk a lot easier, or at least sail upwind between squalls. On my best runs I will always feel really on the edge when sailing into the course square until I get to a broad angle where I can apply and control full power. This may be where the myth of the 'bigger is always better' sail comes from.
I wish I could get all that stuff right all the time, but, alas, it's not always an easy thing to do.

This is just my experience. Your milage may vary.