pacman -Qi | grep -E '^Name|^Installed Size' | sed -E 'N;s/\n/ /g;s/Name\s+:\s+([-_a-zA-Z0-9]+)/\1/;s/Installed Size\s+:\s+(.*)/\1/' | grep -E '[0-9]{3,}\.[0-9]+ MiB

Display packages taking up more than 100MiB on your system.

Most of this incantation is concerned with bringing both the name of the package and its size onto one line. The last grep then filters out the big packages. So you could adjust the size threshold by adjusting that last regular expression.