Hi Sarmale,
Gill length itself is multi-factorial. There is the role of genetics, nutrition, amount of dissolved gases in the water, quality of water and possible presence of an underlying disease/condition.
As long as the axolotls have a good appetite and doesn't demonstrate signs of stress like curled forward gills/curled tail tip, or show obvious signs of illness like fungus growing on its gills, there is unlikely to be pathology contributing to the length of the gills.
If your axolotls are very young juveniles, the rapid growth may play a visual trick such that the body size in comparison to the gills appear bigger. Blueberlin's advise of ensuring good water quality is also most important to ensure good gill health.
Increased dissolved oxygen in the water may cause a normal shrinking of the gills because in terms of physiology, there is less necessity for that large surface area of the gills for gaseous exchange. This is nothing to be alarmed about. It is way more important to ensure there is enough oxygenation at all times.
If general, the younger your axolotl, the more frequently you would need to feed them. The type of food also matters greatly. In general, assuming your axolotl is an adult, i would feed it as much as it can consume in one sitting within 5 minutes.
Temperature, stage of growth, health status and breeding status all determine the amount of food an axolotl requires so there will be some variance between them.
Cheers
