Because it's not percieved as serious, you should make a poll and see how everyone else thinks and percieves it.
There is no reason to portray such vast difference in the carrier of their shadows if it holds no meaning. Coupled with the fact that this arc Zoro and Sanji is fighting a common enemy, Oars, where their display of strength can be correctly juxtaposed and compared. Sanji performs greater feats of strength this arc. There are plenty of examples of this, but one clear such example of Sanji being physically superior is throwing a few hundred meter long chain some let’s say 200+ meters after having sustained grievous wounds. By comparison Zoro trains with maybe 500 000 pounds.
here is the listing of weights of studlink iron chains
You can see that one such chain is about 2 Sanji in diamater, thus 4m in diameter, equaling 400mm. Look at the diagram where it says 100mm. It’s about about 29000kg or 29tons per 27,5m. The height of that building which is the height of the pulley is several Oars high, several hundred meters high. and Sanji is throwing that thing some 200m maybe. He is wounded here and he does it very fast