Often it is how it looks to YOU.
I have seen 1/43rd vehicles used along-side 28mm figures where
the player was very happy with them, whereas I would think the
vehicles too big. Added to that figures that are sold as 15mm or
28mm will be (sometimes) wildly different in size (see attached
picture of two '28mm' figures from different manufacturers).
Sometime the figures are on bases but the vehicles not, which
can make a big difference to the look.
I would suggest you buy ONE of those cars & put it next to your
figures, step back & see if YOU are happy with the effect.