pacsize.py

#!/usr/bin/python3
import pyalpm, re

handle = pyalpm.Handle('/', '/var/lib/pacman')
db = handle.get_localdb()

package_sizes = []

for package in db.pkgcache:
    if len(package.compute_requiredby()) != 0:
        continue
    size = package.isize
    for dep_name in package.depends:
        dep_name = re.match(r'[\w\-]+', dep_name).group()
        dep_pkg = db.get_pkg(dep_name)
        if dep_pkg is None or dep_pkg.reason != pyalpm.PKG_REASON_DEPEND:
            continue
        if len(dep_pkg.compute_requiredby()) == 1:
            size += dep_pkg.isize
    package_sizes.append((size, package.name))

package_sizes.sort(reverse=True)

for size, pkg_name in package_sizes:
    print('%s %.2f MiB' % (pkg_name, size / 1024**2))