The big spikes are from 100k to 150k, and 200k to 250+k. 100k is a bunch of destroyers and frigates, while 150k adds multiple cruisers to everything from the 100k bounty. 200k is the classic Hegemony System Defense Fleet or equivalent with up to three capitals and a healthy mix of cruisers, destroyers, and frigates; while everything from 250k and up has up to twelve capitals and the rest cruisers.
With the price of ships, it is stupid to fight such bounties unless the player is overwhelming overpowered enough to win flawlessly (without casualties). No way an AI fleet would win unscathed, which is why it greatly bends or outright breaks suspension of disbelief.