There is a more lightweight solution in a TorVM - I wouldn't use the
version shipped with Qubes as is, but it can be very readily adapted to
provide a secure tor-proxy-gateway based on a minimal template.
There's documentation at
www.qubes-os.org/torvm.
I asked almost exactly this question some years back - there is extensive
documentation on the Whonix website and wiki covering what Whonix claims
to offer. Look at
www.whonix.org/wiki
On your specific questions, imagemagick is used as part of Qubes, and is
installed even in a minimal template. I've no idea what else is
installed in the Whonix gateway.
I believe that the Whonix WS is intended to provide a complete user
environment, customised for secure anonymous use, (in the same way that
Tails offers a variety of programs). That may account for the large
size.
There is absolutely nothing stopping you from "rolling your own", or
taking the Whonix template and stripping out what you don't want. Make
sure, though, that you don't inadvertently lose essentials. I would
start by cloning the template and trying to strip that.