When Roger died, the number of pirates increased which in turn the number of Marines increased, causing blah blah, we've all heard this.why? why wont they be ~equal?
Oda also blatantly states the newer generation will be stronger than the last.
This also makes sense as..
If Roger was pirate king, but there were thousands and thousands of people who weren't even trying to be pirates or be pirate king, but now all of a sudden this guy figures out the truth and treasure of the world and hides it somewhere, so now all these thousands and thousands of more people start trying to find it so then of course competition, etc is way more heavy this time around.
It was a cake walk for pirates before compared to now. Same with marines