You are comparing weight on them and the ATM has integral Artemis IV, so for comparison yes, you have to give the launchers Artemis IV because that is what ATMs have standard. LRMs mounting Artemis IV lose nothing in the ability to throw other types of LRM munitions, they just do not get the +2 bonus unless its Artemis IV ammo.
Artemis IV gives you roughly +21% extra damage on average during direct fire against targets without ECM, using the -20 column for a sample calculation. Artemis IV is pointless on the Clan's -5 and -10 LRM launchers, has dubious utility on the -15, and has a slight benefit on the -20 in most circumstances. You
can compare damage output and tonnages just fine without integrating Artemis IV to the LRMs, and for a really rigorous analysis one would compare both an Art IV LRM and a regular LRM.
There is an opportunity cost in using the Artemis IV: that's a ton and a crit you can't spend for extra ammunition or CASE or heat sinks, per launcher. Installing Art IV on one launcher means you have to install Art IV on all of that launcher types, even though Art IV is not efficient on small launchers, which generally shoehorns you into using larger -15 and -20 launchers. Often the damage bonus can be useful (especially for IS launchers), but there's not a single design that can't become more flexible by scrapping the Artemis for ammo or other things. (Not necessarily
better, just more flexible)