Perhaps you have git project clones, in which case the size will be much larger. Try deleting the vendor directory, and then run composer install --prefer-dist to make sure you get only the package files and git information. If that doesn't make any difference, I think it's just the file size.
Another discrepancy may be due to the fact that there are many things removed from the vendor directory on the zip at symfony.com. Tests and documentation of vendors are deprived, while the composer does not.
In any case, these are just files. Drives are pretty cheap. I would not worry too much about it.
source share