Magma is hotter than regular flames
Sanji's flames aren't regular anymore. It's evolved to blue flames with actual lightning (for some reason), it isn't like Marco's flames, which are meant to heal people.
It's above any real flame, and it will simply keep growing until there isn't anyone who can handle it.
Queen himself couldn't even react to it, he couldn't even tell what was happening, or mention about how different Sanji's flames were looking, he simply got destroyed.
For writing purposes, i believe Akainu's magma will keep showing how powerful it is even compared to that, especially cuz i believe he is going to fight Luffy, everyone knows that powerlevels can't be treated as a check list 100% of the time to tell that the character that has the most abilities at his disposal is the strongest one
But as for King being stronger than Sanji, Sanji already showed to be better at everything King can do. Anything else falls into Zorotards neverending attempts to make Luffy and Zoro equal, and Sanji a rival to Jinbei, whos getting replaced by Yamato or something