Godzilla x Mothra

After defeating Ghidorah and reclaiming his Alpha Status, Godzilla returns to his and Mothra's secret home where he awaits her return.

When she returns, he is surprised, but overjoyed when she somehow the same size/weight as him, perhaps even bigger.

Due to their countless merging's, Mothra slowly absorbed small bits of Godzilla's DNA until she eventually evolved to be his size/weight, along with the capabilities to reproduce with him, essentially becoming half Mothra, half Godzilla.

Being called fat by the humans DID lead to something good for Godzilla.