Most aspects of the food conversions in Agricola have fairly justifiable real-world bases. Given the game is set in Central Europe around 1670 CE (per the rulebook introduction), it's fairly reasonable from a health standpoint that you need to be able to cook meat to eat it safely (hence, you can only convert animals to food with a fireplace or cooking hearth). Additionally, while you can eat grain raw, you would certainly get more nutrition out of it by cooking it (hence you can use bake bread to turn grain into more food than the default 1:1).
Given that, what foods are vegetables supposed to represent that they can produce as much food as grain when cooked? The pieces are orange, which is evocative of carrots, but that just doesn't make sense from a calorie math standpoint. 1 cup of carrots have a tenth the calories of a cup of grain (45 Calories vs 450 Calories), so the part where you get the same amount of food from cooking vegetables as baking grain with a fireplace (2 each) or a cooking hearth (3 each) seems very hard to justify.
Are there any sources that explain what the vegetables are supposed to represent in the game? Baring that, what foods have the right calorie density and color to be the vegetables represented in the game?