The graphic above shows articles available in UK PubMed Central (UKPMC) - read-only articles: green; Open Access articles: blue - compared to the number of abstracts in PubMed.
For articles published in 2010, using the ~925K articles in PubMed published in 2010 as 100%, UKPMC is about 18% of the size of PubMed, with Open Access articles around 7%.
A key factor to maintain this growth will be for the research community to improve compliance with the UKPMC Funder mandates, to ensure all articles resulting from those funds can be found in UKPMC.
Jo McEntyre
EBI/UKPMC