Cows are your biggest expense but also your income producers because they give you a calf. Finding the proper balance with the number of cows can help a lot. More cows is not always better, but better quality cows is always a good investment because their calves will weigh more and bring a higher price. At 300-400 lbs. those calves are much too small to go to a feedlot, they will get put out on a winter grazing crop like wheat, maybe in your area there's simply not many buyers needing light weight cattle. In the sale ring perception becomes reality in a few seconds, if the buyer thinks it looks sick they won't bid, if they think it's a crossbreed they don't like, they don't bid. Raise what the buyers want!
Something for you to consider.....the next person in the supply chain (the cattleman that now owns your calves) will likely make more money than you did when they sell them to the feedlot. He will do all of the value add things that you didn't do, weaning, castrating, vaccinations, etc. With the high grain prices the feedlots are paying up for the larger yearlings. In the past larger weights were discounted but that's not the case right now.