Axolotls gills size alters depending on 4 things,
Oxygen levels in the tank = more oxygen = shorter gills,
Water quality - Its gills will shrink and look wierd, and it will use its lungs more to gain oxygen - can eventually lead to metamorphing which could kill the axolotl.
Genetics - some have shorter gills
And finally if the axie is with tank mates = can nibble gills making them shorter.
I know i have put 4 things, but i swear im missing something ( writing this half asleep)