Plants make their food in their leaves. And need sun for this process, so smaller and fewer leaves means less sun for photosynthesis, and it also means less food to nourish the plant.
If haploid didn't occur on gametes, it would occur on diploid, which would increase the number of chromosomes, and the child would have a lot of them.
So, what would happen if gametes were not haploid is that B. the offspring would have double the number of chromosomes it is supposed to have.