I don’t think it’s fair to compare an old double-gauss lens design to the modern high performance 50mm designs we see these days. You could make the same comparison between the Sigma 50mm f/1.4 Art and the Canon EF 50mm f/1.4 USM and ask Sigma “what are you doing? Your lens is huge!”. If you took the trouble to shoot them both wide open and look at the results, you wouldn’t even need to zoom in to 100% to see the difference, the 50mm Art is in a different league. The Canon 50mm EF f/1.2L actually resolves pretty similar to the EF 1.4 at equivalent apertures.
A more interesting comparison would be to other mirrorless 50mm designs, but as the major manufacturers have only released 50mm f/1.8 lenses so far, we don’t yet have a fair point of comparison.