No, not static units. After some more experiments, I believe the extraordinary delay in final elimination only happens in the case of surrender of enemy units on the defence, irrespective of enemy (original) unit size. For weak assaulting enemy units, when they surrender defenders they do it quickly.
Once, a completely surrounded Infantry company reduced to 1 man held up a couple hours before finally surrendering. It appears that enemy units 'negotiate' surrenders: my unit icons have the blue fire indicator on, but no firing based on the sound effect, and they stall there, until the enemy finally gives up.
Nonetheless, it does appear to me that the reduction of units follow a kind of convex curve instead of linear: the smaller force becomes, the fewer casualties it takes under the same fire power and time. This might be realistic, though it is a bit troublesome that the few supposedly dispirited soldiers left often go on guerilla warfare after they flee.