Question of weight?

Weight is often mentioned here as a factor for speed, but I do not completely understand what that means. Me personally m40,189cm always weighed between 90-96kg since I was like 17yo. I only got under 90kg in a phase when I was running 10k like almost every day and ate quite restrictive. My approach to sport was always mostly that I do as much as it needs so that I can eat and drink as much of whatever I want and still looking fit and healthy. I absolutely avoid the nutrition topic and will always enjoy drinking and eating. I would never starve just to maintain a six-pack. However, now since I started rowing on the C2 last October like for 4-6 times per week my weight increased to 98-99. I always said that I do not want to weigh over 100kg. I am not lifting weights at all.

Is it normal to get heavy just by rowing? Will it reverse after reaching a certain level? What would be the perfect or most efficient weight for a rower my size? Would I automatically be faster, if I reduce my weight like it is the case in running?