I'm not entirely sure I understand what you mean by "modern fantasy" here.
Do you mean literature?
Film?
TV shows?
TTRPGs?
Something else?
I haven't done an extensive survey, but my initial impression is that as a general rule there is a strong correlation between strength and size in modern fantasy. It may not be perfect, but it certainly exists.
All of the above.
Fantasy in modern film, books, comic, video games, TTRPGs, and TV doesn't push a large correlation between size and strength until you get to extreme sizes.
And that point Size gains a correlation with low speed or dexterity.
From what I've seen, when size becomes a factor in a character's strength it comes at the cost of being a clumsy oaf. So you only see it in actual Large size creatures like Giants and Minotaurs. Human sized beings don't get much of a size boost in strength and have been shrinking in size since the 80s.