Frozen food is usually more convenient of the equivalent fresh vegetable. First of all they do not have waste, that sometimes is high for many vegetables. Then they are washed and blanched, and this passage reduces the weight.
For example, to have one kilo of cleaned and cooked spinach, you need two kilos of raw spinach to clean. So, with frozen food you just pay the net weight you are going to consume, and you also save time. Moreover the price is stable, as it doesn’t depend on the market and on the season.