Though I'm not discounting that it may in fact be dimmer:
Larger gaps alone implies dimmer, but this can be accounted for by having the pixels put out more light. Having larger gaps on a display doesn't mean it's dimmer, as the pixels have likely changed characteristics in the process of shrinking them.
(It’s, by the way, not that the gaps got necessarily bigger, maybe a little, it’s just that they didn’t shrink either and, with more pixels, there are also more gaps.)
Larger gaps alone implies dimmer, but this can be accounted for by having the pixels put out more light. Having larger gaps on a display doesn't mean it's dimmer, as the pixels have likely changed characteristics in the process of shrinking them.