As well as the common usage of grow to increase in size it can also be used to indicate a gradual or progressive change of state for example

As the night fell it began to grow colder.

Having said that using the phrase ‘grow’ smaller is probably best avoided as it is a bit ambiguous as it is not clear whether it means they actually shrink or they just grow more slowly, reach a smaller maximum size or indicating a trend in in the population as a whole over time.

Although this is often the case with headlines and often the ambiguity is deliberate in order to make a story sound more interesting or sensational than it really is.