Usually when 'oversize' wire is specified it is to allow a longer length, for reasons of voltage drop or because the location is hot and ventilation is poor. I suspect if this is all co-located and well ventilated. it could easily be reduced a bit.

The MCB vs RCBO thing is more interesting, and depends how reliable earthing might be and if the designer is assuming it may be a TT supply or a higher risk location, like supplying sockets outdoors. A primary side RCBO offers no protection to the socket users of course- that would need one on the secondary side - so really we are looking at detecting damage or damp in side the transformer case causeing a fault to the case that is not enough to trip an MCB.

It looks like a 'one size fits all' diagram that is safe when kit is installed almost anywhere - it may well be that in very specific situations that can be relaxed quite a lot without any risk.

Mike.

I think I know why external cable are 4mm2. According to Emergency disconnector datasheet the terminals can accommodate wires min 6mm2/10mm2 depending on the wire type so it's not perfect but I understand the idea of the size increase. This 110V circuit is going to be fitted in PRR so it maybe worth stick to RCBO then however shouldn't it be something higher than 16A? 

I think I know why external cable are 4mm2. According to Emergency disconnector datasheet the terminals can accommodate wires min 6mm2/10mm2 depending on the wire type so it's not perfect but I understand the idea of the size increase. This 110V circuit is going to be fitted in PRR so it maybe worth stick to RCBO then however shouldn't it be something higher than 16A?