If you look at Figure 3 (and section 3.2.1), you'll find they're reporting the n...

		Stoffel on Aug 18, 2014 \| parent \| context \| favorite \| on: A surge of p-values between 0.040 and 0.049 in rec... If you look at Figure 3 (and section 3.2.1), you'll find they're reporting the number of papers in percentage of the total number of papers. So, relative size. The increase given in the abstract is that between 1990 and 2014.