Richard and Robert Hammered Halfpennies

At last night's detecting club monthly meeting a couple of coins were on display that I'd never seen before. Both were hammered halfpennies, the first a Richard 2nd and the other a Robert 2nd of Scotland. The fact that so few halfpennies seem to be found got me thinking about their rarity. In the past I always thought that their small size may have made them less likely to be found by detectorists but now I'm questioning that theory. What are your views on this? Apologies for the poor photos.