Why do people say to use heavier bullets in a rifle that has poor ballistics? Wouldn't you want a lighter bullet so that its fast and shoots flatter further? For instance I had a 25 wssm that everybody said shoot the heaviest bullet you can out of. I found that the gun shot better with lighter bullets than heavy. Any thoughts.

The bullet weight is up to you. But for comparing the BC's of the bullets, generally your heavy for caliber bullets will yield the highest BC for longer range shooting. If you do not plan to shoot 400+ yards, then BC really doesn't matter that much. Go with what shoots the best. The problem in the WSSM case is, the round needs to be really short, and a heavy 25 caliber bullet would be hard to fit in the magazine.

If you run the numbers on a light weight fast bullet and compare the drop and wind drift to a heavier bullet starting out slower, the heavier bullet will begin to have less drop and much less wind drift than the lighter bullet does, b'c the heavier bullet is more stream-lined, and does not bleed off speed as fast as the lighter bullet does.

what chad said but let me add. less wind drift with heavier or lets say higher B.C. bullets start immediately out the gate but with more drop, not until you get about to 600 yds does the higher bc bullet start to drop less but with much much less wind drift. its way easier to compensate bullet drop, any basic calculator gives you bullet drop, figuring wind drift is the harder part so less drift helps you a bunch.
its also been said that the best accuracy of a gun is achieved when the heaviest bullet is chosen right up to the point of almost exceeding the reccomended twist rate. that fine line of perfect twist matched to bullet weight.
if a bigger caliber is always better wouldnt the biggest bullet for said caliber also hold true ? i belive so. i try and run the heaviest bullet my gun will shoot. and heavy bullets tend to penetrate better without the need for bonded bullets.

^^What they said^^

Plus look at heavier bullets equaling higher BC for the most part. And also, BC is the ability to maintain velocity. It may leave the gate slower, but it will retain a higher percentage of the MV if it has a good BC.

This is apparent in the 308 Winchester. Load a 168 gr SMK @ 2750 fps + and it will not be supersonic to 1000 yards quite often. Load the same case with a 175 gr SMK @ 2650-2700 fps and it will be supersoic well past 1k.