Generally, each round should be at least 2x the size of the last one, or larger. The simple reason is you want each round to be at least 2x the share price and valuation of the last one. Hence, for the same amount of shares, you should raise double the amount in each round.
So most A rounds are >=2x the size of the Seed round, most B rounds are >=2x the size of the A round, etc.
If the company doesn’t really need any more money, and the round is opportunistic … they may not want the dilution. They may sell 5% in the C round, instead of 20% in the B round.
So the story isn’t always clear from the outside.