Radak: Also I will increase the fire. Because it said "Dur ha'Atzamim Tachteha" (5; burn the bones under it), Hashem says 'also I desire this!' They increased evil - I will increase vengeance against them and add wood to the bones to increase the fire. Wood catches fire faster than bones, which are hard. I will make a big fire so the meat will be consumed. Also the bones which are hard, will burn from the strength of the great fire.

Malbim: Aside from the fire from outside, I will increase the fire under the pot, i.e. the sword, hunger and plague that consumed in Yerushalayim - "Hineni Mesev Es Klei ha'Milchamah Asher b'Yedchem... v'Asafati Osam El Toch ha'Ir ha'Zos" (Yirmeyah 21:4).

Rashi: It is a great fire.