Does it make sense that the gain on the fronts would have to be set 30% higher to get the same output as the rears, even with the same RMS ratings? And is that what I want to do?

Yes.

If they were the exact same brand and model of speaker, I would worry. But if they're a different brand or model of speaker, then it's normal to get different amounts of volume out of them.

The RMS rating is not an "expected volume" rating. It's a "how much power can I put through this speaker before it distorts" rating. The volume is determined by different factors, such as speaker cone size for example. Some places quote a number called "efficiency" which is supposed to indicate how much of the amplifier energy is converted into sound waves. When comparing two similarly-sized speakers with similar specs, the one with the higher efficiency number will be louder.