It is very un-eco to hunt youngs with 30 HP due the over DMG. The bigger the maturity the less overkill %wise and the more eco you can hunt.
To take the most extreme example:
Daiki young 30 HP : 2 shots with korss 400 of 25 DMG avg = 60% Overkill = 60% weapon decay/ammo lost.
Daiki Stalker 830 HP = 34 shots with korss 400 of 25 DMG avg = 2%-ish overkill
PS: Yes I know that you'll have armor/fap decay with bigger ones, but you you get the picture.
hm, lets count it maybe in other way, assume you do 3 HP dmg / pec and MA returns 90% loot (3 and 90% are ofc some approximation, if anybody knows better numbers use it):
Daiki young 30 HP : 10 PEC to normal kill, 6 PEC loss -> 60% overkill, 1 PEC loss -> MA, equals 10 PEC to normal kill + 7 PEC loss
Daiki stalker 830 HP: 2,76 PED to normal kill, 5.5 PEC loss -> 2% overkill, 28 PEC loss -> MA, equals 2,76 PED to normal kill + 33.5 PEC loss
now you can multiplay loss by 16500
also as you said bigger ones need more fap/armor, and markup on gun/amp if you use L ones
on the other hand, daiki youngs do not give anything with signifficant markup, maybe big ones can do to compensate a bit loss, but still seems that young ones are better for mission