The squid doesn't need to resolve the little sea creatures in order to detect a whale. It only needs enough resolution to resolve a huge number of the little lights into a whale-shaped blob. For that, it needs an eye with the biggest possible aperture and sensitive sensors. The increased resolution may be an additional benefit of the better light-gathering ability of a big eye but isn't necessary.

The article says the researchers calculate that the squid eyes they measured are the optimal size for seeing something far away and massive. Why isn't the biggest possible eye that can be grown the best? Is there some other constraint besides the mechanical structural issue or risk of damage, etc.?