Actually, in lovebirds the males tend to be smaller, not the females! Females are supposedly a bit larger, broader, and when sitting have a wider stance of feet than the males... you might want to take a look at them when they are sitting on perches next to each other to see who has the wider stance...
However, considering a "new mutation", the longfeather (much like the english budgie, but not quite as established, being that a clutch can yield normals, intermediates, and longfeathers all in one) is larger than normal peachfaces, so if the male had "longfeather" (aka Standard Type roseicollis) in his genes then he could very well be larger than a normal female.
http://users.accesscomm.ca/parrot/articles/LFT.htm
http://africanlovebirdsociety.com/genet ... ather.html
http://www.birdhobbyist.com/parrotcolou ... ngfpf.html
Although considering, I'd say that longfeathers are probably pretty rare in the states, so my guess is that the larger one is female, and she's being protective of her nest... lovebirds, especially and particularly, the peachfaces, are very aggressive! They will attack another bird over 100x's their size if they see fit.
You might want to consider getting DNA tests done on the pair to ensure they are male/female. However, it really is my guess that if you have a male/female pair (and not female/female, which does happen occasionally with the birds acting out like a true pair) then it is the female who is protecting the nest... and females are known for bullying around the males!
You may get some better advice in the lovebird forum if you ask there!