In the last couple of weeks i have noticed my fonts suddenly getting much bigger than originally set.

It seems genesis have added a fix:
/* Chrome fix */
body > div {
font-size: 1.8rem;
}

that seems to be messing everything up

so in my child’s theme i have added:
body > div {
font-size: 1.4rem;
}

which seems to have fixed it – but its a worry