Think of it from the casino's POV; They have a 2.7% advantage on every number, so the more numbers you bet the more profit they make (and the more you lose), just as if you have found numbers which give a positive advantage then the more numbers you bet on (assuming they all have an advantage), the more you will profit.

In reality is slightly different - after the ball is spun - there are about 18 numbers where the casino has the advantage, about 4 around zero and 15 where player has the advantage. Only of course usually we not know who is who...

But imagine, that we have the deck of cards with 15 positives 4 neutral and 18 negative cards. What is a difference how much we take cards?