I've just checked through my dates from 1882 (F114, F115) through to 1894 when obverse R was used and, like Paddy, my 1890 and 1891 pennies all have the fatter chin, but 1882 to 1888 and 1892 to 1894 have the normal chin. It seems strange that this slightly different obverse was used for a 3 year spell in the middle of a 14 year run.
I've asked Michael Gouby for his views.