Bimodal frequency distribution of size of protein loops
A curve of this type, showing a bi- or multi-modal distribution,
  suggests two or more constituent species
  in the population. In this case this is borne out by further analysis
  in the paper of different types of secondary structure components
  anchoring the loop, the peak at low numbers of amino acids reflecting
  the loops connecting β-strands in anti-parallel β-sheets.


The second frame (B) of Figure 2 is shown below (dashed line):



Anti-parallel β-sheets are abundant in proteins, and in many — although not all — cases adjacent strands are connected by short (four- or five-amino acid) loops such as the β-turn, the β-hairpin or the β-bulge loop. 

Repeating small loops are especially well illustrated in β-sheets displaying β-meander:



The β-meander motif from the outer surface Protein A

Examples in larger domains of the β-barrel and immunoglobulin type also show some larger loops.



Beta barrel of green fluorescent protein, showing loops at end of strands



Immunoglobulin domain of PDB 5IML showing β-sheet with loops connecting strands