They had to make him looked as buffed as possible to make it believable that he could defeat King Ghidora. In earlier movies he never beat King Ghidora alone. In their first encounter he had Mothra and Rodan fighting with him and the next time it was Godzilla and Rodan, this is the first time Godzilla by himself faced King Ghidora. And King Ghidora is much larger and overall more powerful so being the underdog making godzilla look as buffed as possible is for the same reason Rocky had to look as buffed as possible in Rocky 4.