I’ve seen comparison pics online (don’t recall whether it was from a Google search or from a post here on LAF).
The Mantic halflings are more like three-quarterlings - quite large in pretty well all their dimensions (head size, hands, overall bulkiness) as well as height. It’s almost as if they designed them digitally, then forgot to set the correct size when they cut the production moulds.
I have a sprue of Wargames Atlantic halflings and they’re much closer to what I expect for 28mm halflings.
Summary is, not really compatible with each other and the Mantic figs might even sit oddly next to most 28mm figures.
Other folks who have the Mantic ones might be able to provide more details.