Hmm, good reply Transitive.
But I do understand how this works, though I'm sure your explanation will help others. I should have written in more detail to demonstrate what I mean, as you did with your example.
If overkill was unmanaged by the AI targeting systems, it would play a significant factor in how ship-size affects the effectiveness of focus firing, but thankfully, a large part of overkill is anticipated and handled by the AI if not force-re-targeted on the FF-target ship.
IE, if you select your LRM blob and tell it to Focus Fire on that Cobalt Light Frigate, your LRMs will start targeting other ships while the killing volley is in the air, unless you reselect the LRMs and target the Cobalt again. Call that "Force Retargeting of FF Target", or just "Force Retargeting".
There are quite a few difficulties in trying to generalize about the effectiveness of Focus Firing because many aspects of its effectiveness change situationally. It may be that our mini-debate here over target size isn't generally resolvable, but at least we can list the factors that we can determine.
As you point out in your example (and as I had noted, but too briefly, above), larger ships will enjoy a longer period under their highest shield mitigation, proportionally. And you correctly determine that this is a clear and constant ++ for larger ships. However, it is generally a tiny difference.
Recalculate your example for other more typical DPS vs. Hull situations. Try 500 hull vs. 750 hull @ 10, 50, 100 DPS.
Now, its true that we can come up with even more generous examples than yours, but these involve TINY ships, or very damaged ones. But as I noted, flocks of tiny ships (far smaller than we have in game) would break the effectiveness of Focus Fire anyways, for many reasons.
The debate is really whether the tiny bonus enjoyed by larger ships due to shield mitigation giving them a (very slightly) longer window of damage absorption offsets any weaknesses larger ships have to focus fire. I don't think it does. You do. I'll try to come up with an example....
Proposition: Larger Ships are better than smaller for surviving Focus Fire.
Negatives
Less Attacker Retargeting
Less Attacker Turning Time
DoT cruiser skills more useful
Less Attacker Overkill
Positives
~.01% - 1.00% more damage done while fully mitigated
Longer time to enjoy any recharge/repair
........
So maybe its debatable, but I really don't think its any benefit to have few+large ships in a Focus Fire situation, where I do think the benefits of many+small ships are clear.