AGnu - I pondered a bit about this upthread. AFAIK, both of them were very conscious of their weight. It's known that Reeva purposely lost weight to further her career and OP when interviewed would frequently talk about being on a strict diet to keep his weight down.
So, in that sense, it seems likely that they would eat something fairly light, early in the evening.
I am not sure if the State think they ate later then they said, or that Reeva herself ate later. I do wonder if there is any rubbish or leftovers found in the house that would indicate exactly what was eaten.
Also, I believe at least one witness testifed that, at the crime scene, OP was retching but not vomiting, which (if that witness is to be believed) would demonstrate that he had an empty stomach.