Which citation style is the most concise and has the least impact on word count?

Coming back to this question when writing a grant with very strict word limit, I discovered that the most concise citation AND bibliography style must be the Science (without titles).
Example:

1.R. Hisakata, S. Nishida, A. Johnston, Curr. Biol. 26, 1911–1915 (2016).

You can find the citation-style-language csl file in the official csl styles github repository, that you can download and use in Zotero, Mendeley, RefWorks, etc.
If you're an EndNote person, see the style file in the most popular answer to this ResearchGate question

As @Adam Bosen commented, you will be hard-pressed to find a more compact in-text citation format than IEEE. A single citation is written [1] and several citations [2-5].

The bibliography is then listed in numeric order.