here the biggest and smallest products of Orrenstein & Koppel

No, it´s not the smallest excavator. They have (or had?) compact excavators as well.
But it´s a very good picture.

Correct me if im wrong but the smaller of the two is an RH4, the first hydraulic excavator built by Orenstein & Koppel.
O&K sold Macmoter and then laterly Schaeff compact excavators under its own name until the demise of the O&K name and the creation of New Holland Construction.
At least the name still lives on with Terex.

No, what you are seeing is indeed an RH5 and it was the first fully hydraulic excavator manufactured in Dortmund. My Dad was one of the first ones to put his hands on one of the prototypes some time around 1965 or 1966 when he worked for Hochtief in Essen, which is just around the corner from Dortmund and was a premium customer for O&K.

At that time, the RH5 was O&K's only excavator which is why they wanted to show the dimensions between their largest and smallest machine on that photo.