When feeding an axolotl a tropical fish bucket, the bucket becomes a water bucket. If this stack is oversized, then the count is ignored; the resulting water bucket stack will always have a size of one. I would expect only one tropical fish from the bucket to be consumed, leaving 63 tropical fish buckets and 1 water bucket.

Steps to reproduce

1. Spawn an axolotl.
2. Obtain an oversized tropical fish bucket stack: /give @s tropical_fish_bucket[max_stack_size=99] 99
3. Feed the tropical fish bucket stack to the axolotl.
4. Observe that the result stack is 1 water bucket rather than the expected 63 tropical fish buckets and 1 water bucket.