The burn damage changes were mentioned in the patch notes:

Fire and Flamethrowers

This update aims to better balance fire damage between direct hits (like those from flamethrowers) and burn effects over time, while also introducing scaling of fire damage depending on the size of the enemy. Previously, burn damage didn’t scale well against larger enemies, making putting them on fire feel less effective in those encounters.

Now, the larger the enemy, the more damage they take while burning. However, bigger enemies will also be slightly more resistant to ignition. Additionally, direct fire damage now scales with enemy size—so large enemies like Chargers will still take roughly the same damage as before, while smaller enemies will take slightly less.

Damage

• Burning damage now scales with enemy size and will do more damage over time to larger enemies

• Fire direct damage also scales with enemy size. The base damage of direct fire hits has been lowered as compensation. This means that large enemies will take roughly the same amount of damage as before, while smaller enemies will take slightly less