Having had some thought on this is in my shop I think I know partially some of the rather dull answer….
Reg Digestives are 0% vat come in a slightly larger pack.
Choc digestives (along with all chocolate coated biscuits) are 20% vat as HMRC says that they are a luxury item.
So my guess is that to get the same or similar profit margin the choc digestive has to be a smaller biscuit and smaller pack size.
That coupled probably with the quirks of the machines that coat the chocolate or manufacturers the biscuits would determine what size the slightly smaller chocolate digestive has to be to fit within the margins of manufacturing.
Regards vat this is also why Jaffa cakes are cakes (because cakes are an essential, not a luxury!) and chocolate cookies like Maryland are charged at 0% so are generally quite a cheap biscuit when compared to a chocolate digestive.