The dress was too small in the shoulders

The dress was too small in the shoulders. Why in this sentence does the adverbial phrase (in the shoulder) modify the adjective (small), and not the verb (was)?

What can I read for a clear answer to this question?

The locative adjunct in the shoulders does not modify small, neither does it specify the location of the dress or the situation as a whole. It serves to indicate which part of the dress was too small. It is approximately equivalent to:

The shoulders of the dress were too small.

The relevant question to the sentence given would be:

Where is the dress too small?

A discussion of similar cases may be found in CaGEL (the Cambridge Grammar of the English Language) pages 681-682.