It is a particular case of gijinka and mecha, where women in manga/anime style are representation or personifications of weapons or military equipment.

Particular case: MS Shojo

It is when a gundam (giant robot, mecha) acquires a human feminine personification. This personification must be so that the armor is the "clothing" (or armor) of the woman.

Special case: Kantai Collection

In its respective article explains why they are pseudo-giantesses and why it is popular within the GTS community: