In many cases, this is not true, infact there can be an advantage in using an oversized motor.
For example, from a manufacturers data sheet (attached) I see the following:
100% load 89.2% eff
75% load 89.7% eff
50% load 90.7% eff
So, if I have an 11KW load, I would be most efficient to use the 22KW motor at half load, followed by the 15KW motor at three quarter load and least efficient to use the 11KW motor at full load.
The oversized motors would run cooler, and therefore would last longer as well. The disadvantages of using an oversized motor would be that the start current will be higher and the capital cost is also higher.
I find the same if I compare the 30, 45 and 55KW motors, and others as well.
As I see it, the use of an oversized motor does not waste energy, it actually can save energy!!