The lake reed definition in flora_newherbs.txt has [ROOT_SIZE:XS]. On testing, it doesn't look like XS is a valid value here. Instead, each lake reed root produces the same as if it were ROOT_SIZE:M, the default option. This makes lake reeds much bigger than they should presumably be, and in practice gathering a bunch of reeds for flour and vegetables over the winter is very effective. I dropped them down to ROOT_SIZE:S and ROOT_QUANTITY:1 in my own game to get it approximately aligned to what I figured was intended.
I get different results. XS is a valid value, and takes effect as it should - at least on my end.
If you use metric units one lake reed root should weigh around 50 grams in character's inventory, which is the intended XS.
If they were default (M) they would weigh around 150 grams. If you change it to S they will weight around 75 grams.
Haven't spent too much time on actually calculating if the average harvest is too low or high IRL, but can't find a proper bug here.
And lake reed roots are packed with a lot of carbs.