as a monster dies the skill line can get a 2x or 5x reward as normal skill progress line (system panel)
the smaller the monster the more "bonus roll" happens for ped spent
this boosts skill gain per ped
another optimization is the HP/Level parameter (small monster = carabok, big mosnter = flesh rippers)
of course WORST skill/level ratio are world boss ..... but those are not killed for skills ....
i would add "think to the TOTAL" return of the monster and dont think to the TT return.
we can not control anything in the game but the long term MU in MU out and the skill/tools parameter
you said you kill spiders, i assume that are not 5p1d3r robot level 15 but level 52 araneatrox on caly.
those with a 60 combat 33 looter and probably 35 evader are a BAD target because you are often critted and almost always hit rising defense cost, commanding a costly armor, or ask for fapping to keep HP up, allowing monster to regenerate (they regen fast).
if you want to kill such big targets it is more important DPS than eff... the regen would kill weapon stats.