Hi, You might find it starts growing again or, if it has stalled for some reason, it might send another out when it's ready which will be back to normal.
The reason I say this, is that I have a Phal Mini Mark with a leaf that is half the size of the rest. The growth of this leaf coincided with the growth of 3 flower spikes and so I put it down to the plant using energy for other things. It started growing another leaf once the flowers had finished and, as far as I can tell, it's going towards the full size of the others