So basically Luminus has helped point out that ALL the Borg vessels in VOY were smaller than the one we saw in TNG, right? None were as massive as that Assimilation Cube?
So then they were ALL smaller and weaker than in TNG, explaining discrepancies in why VOY had an easier time.
I hate you. Grrr.
Don't worry. The Assimilation cube on the chart is 3000M, so what you have actually helped point out is that this cube is in fact the variant-2 cube, not the one in TNG. The tactical cube is also 3000M, but the chart incorrectly displays it as 1500M. The 3KM variant-2 is the one in First Contact
which still took an entire fleet and Picard's inside knowledge to defeat, so no, this doesn't explain the discrepancy at all.