Yes to all of what you asked.
With regard to the question:
Could the growth be seen as proportionate to size?
It's not best constrained that way. One take on that concept is to constrain on consumption. That was my original approach. But there are significant problems in the early stages with that approach. There are really two stages of growth. Geometric growth of sexually immature fish and the consistent annual growth of sexually mature fish (> 9"). I model both.
FWIW, the first year is less important than subsequent year's. What makes trophies are consistent gains year after year. Where it gets really tough is 3 or 4 years down the road.
As far as I know, a 2 lb Female LMB can make 1.67 lbs of growth in 1 year in your climate and a 5 lb LMB can do the same. But for the 5 lb LMB it is only a 33% gain while for the 2 lb LMB it is an 83% gain. The consumption for the 2 lb LMB making that increment is a higher proportion of its initial starting weight than is the consumption of a 5 lb LMB making the same increment. Throughout life, an LMB's potential for proportionate gain declines even though it's potential for annual weight gain remains fairly constant.
