What to record
For every size observation, capture the fish name, size label, mutation, displayed income, and current update date. That is the minimum needed for a useful comparison.
Does bigger mean better?
It may, but the official description only confirms that sizes exist. It does not state the formula. The site will publish a multiplier only after matching fish can be compared reliably.
Collection versus progression
A visually large fish may be valuable to collectors even if another target is more efficient for money. Keep collection rarity and progression value as separate rankings.