In the 'Introduction' to James Thurber's 'My Life and Hard Times,' I came across this sentence: 'He gained in depth and skill as he went along, of course.'
I don't get how 'in depth' is used here. The way I understand this is, the words 'in' and 'depth' are used separately and not as one single element. Then, shouldn't a noun/pronoun follow the verb 'gained?' For example, 'he gained nothing/something' or, 'He gained in India, nothing.' The presence of 'depth' and the absence of a noun confuses me a lot.
Or does the given sentence mean that he gained a lot and also gained skill as he went along? In that case, wouldn't it be a case of violation of the principle of parallelism? Because the verb 'gained' follows 'in depth' (an adverb, meaning thoroughly) and skill (a noun) but not a noun and noun or an adverb and adverb.