The style keys
large only change the default separation between vertices, but they do not rescale the overall diagram, and this should work in all compilers. (Note: At the moment it does not, it only works with
lualatex, see https://github.com/JP-Ellis/tikz-feynman/issues/35#issuecomment-258429193. So either use
lualatex, or the suggestion at the bottom of this post.)
As a result, if the position of vertices is specified explicitly,
\vertex (a) at (0, 1);
or if you specify the separation explicitly,
\vertex [right=1cm of b] (a);
large keys will have no effect on the placement of these. Since you haven't provided the actual diagram you are drawing, then I'm only speculating that this is the reason.
If an overall scaling is really desired, it is possible to do so with the appropriate TikZ key, which can be passed to
\feynmandiagram. In particular, use:
\feynmandiagram[scale=2,transform shape][horizontal=a to b]
where the first set of brackets are the TikZ specific options, and the second set are the TikZ-Feynman options. you can change the
2 to an appropriate value.