Or, more correctly, it violates the laws of evolution as we understand it. There are tradeoffs that are made for offspring in nature. The general rule is that the quicker it takes for a creature to develop, the less complex it can be. A fly, for instance, is capable of all fly behavior within 24 hours, give or take, but fly behavior is very simplistic. An intelligent species requires far more time (humans take ~23 years to fully develop, physically and psychologically). Thus humans need to be protected as babies.
This species would need to be protected as well in child form, something which being considerably large than the adult would hinder. Then there's another thing as well. You see, the reasons humans grow larger to adults is that adults are better at doing things, like crafting objects, and/or defending themselves. That is, an adult human is physically superior to a child human, thus a child grows into an adult.
Accordingly, in order for a large creature to turn into a smaller creature, it must be advantageous. Therefore, for your alien species, a small size is better than a large size. This, itself, is not problematic - there could be a myriad of reasons why smaller is better than larger (mostly external, i.e. predators or the environment). However, this is when evolution is violated - if the smaller, adult, is better than the larger child, why is the child large to start with? That a complex process which consumes incredibly large amounts of energy, all to be discarded, with no benefit, as we know, because the adult is smaller.
It's not impossible for an alien to grow during a life cycle for specific reasons. But a child-to-adult growth large-to-small would not be selected against a child-to-adult growth small-to-small when small is advantageous.