The point was that insects cannot be enlarged much, because their skeleton, joints, respiratory system and circulatory system are optimized for small sizes and they no longer work at big sizes. Also their nervous system becomes too slow at big sizes.

To be able to live, Mothra would have to only look from the outside as an insect, but to have internally a structure like a vertebrate (and even that is limited in the achievable sizes).

In general, when analyzing the differences in structure between arthropods and vertebrates, the solutions used by the former are better at small sizes, while those used by the latter are better at big sizes. Even if there is an overlap in size between the biggest arthropods and the smallest vertebrates, neither vertebrates can be made as small as the smaller arthropods nor arthropods can be made as big as the bigger vertebrates.