That's sort of the point. When the M61A first showed up in Handbook House Davion, it wasn't a compact Grenade Launcher. It fired Class B ordnance, which makes it a standard grenade launcher. It didn't have as many rounds as a standard GL, and slightly less range, but it had more ammo and a bigger explosive than a compact.
But the M61A was only 6kg in weight. Which is about a 30% reduction in weight of the component parts, which might make sense since you don't need the full weapon..and the increase in price over the component parts was only about 25%.
But if you look at the Mauser 1200, the change of weight was a 20% increase. The cost adjustment was around 200%.
The Mauser 960 was similar.
The M61A just stood out. It was incredibly cheap compared to all the weapons of a similar type (I think the Mauser IIC has something like a 400% increase in cost) and it was incredibly lightweight compared to its contemporaries.
ATOW thankfully fixed that, and moved it more in line with the rest of the weapons (whether or not you agree that they should be that heavy, but atleast its consistent now) with regards to cost and weight.
Of course, now that the Mauser IIC has a standard grenade launcher instead of the automatic one, I wonder why its still encumbering...