What is the meaning of this sentence: "What they lack in size, they certainly make up for in importance" [closed]

I encounter this sentence in the bellow paragraph while reading the article here: https://www.openaccessgovernment.org/artificial-intelligence-hr-and-recruitment/64358/

Human resources is arguably one of the most important departments in any reasonably sized organisation.

"What they lack in size, they certainly make up for in importance".

Thanks,

'Human resources' here means "The department of a business or organization that deals with the hiring, administration, and training of staff".

So the sentence means this:

The department of human resources in a reasonably sized organization may be smaller in size than other departments, but it's more important than other departments.

Structurally, what they lack in size is object of the preposition for, which can be marked with a pronoun it:

What they lack in size, they certainly make up for it in importance.