I have a pair of blue tits now with chicks. The female can go in and out of the nestbox no problem. The male is struggling to enter and has to pass food to the female as he hands off the entrance. Is this normal? I have seen him go in twice, so I know he can fit. Even the female on incubation had to leave the nestbox in order to be fed by the male. The entrance is 28mm, not even standard 25mm! Blue tits are getting bigger in Kildare, Ireland?! As the chicks get older I am worried as they need more feeding by both parents.
As Luke says, it's perfectly normal.
It could also be there is insufficient room in the nextbox for him, her and the brood, but the important thing is, he's sticking with them, and that's what matters.
A word of caution, ensure you keep a distance from the nestbox, any suggestion of a threat could mean the parents abandoning the nestbox, and what sounds a perfectly healthy brood will die of starvation.