Unless management would charge a new tenant more per month for that apartment, why would they care which apartment you live in? I find it bizarre that they would ask you to justify your desire for more living space.
Are there possibly other, equally good, tenants in the building that are also asking if they can move into that apartment? If so, management could be trying to decide which tenant to give it to. In that case, they might decide to give it to the tenant with the best reason/need for the larger apartment.
Ask them which apartment they would prefer to live in.
I just realized that I haven't lived in an apartment since 1975!