This is only true if you're guaranteed to get one boulder every four tiles mined. If it's actually a 25% chance of getting a boulder for each tile, then the odds are not at all in your favor. It's technically possible, under such a system, to mine out a 10x10 room and not get a single usable stone.
Strictly speaking, the odds are exactly as much in your favor as they were before.
No, not really. Prior to the change, a legendary miner could dig out a 10x10 room and get 100 boulders guaranteed. After the change, a legendary miner could dig out the same space and it's technically possible to get no stone at all, if it's a 25% drop chance per tile. Anyone who plays an MMO can tell you that an item having a 1% drop chance doesn't guarantee you'll get it after 100 kills. =P
As I say, if it's a guaranteed boulder every four tiles, then it's no problem. If it's a 25% chance, then it's luck based and there's no guarantee you'll get as much stone as you want.