Comparison of large adult with juvenile (images not to same scale). Both are frond dwellers with a slight brownish tint on the sole, stronger on the adult.
1: anterior limit of gills.
Relative to shell size, the gills occupy less of the perimeter on the juvenile, 68%, than on the large adult, 84%.
The perimeter of the adult is four times longer than that of the juvenile but the gills need to provide oxygen to a body mass about 64 times that of the juvenile. The size of the gill leaflets does not increase at the same rate as the body as a whole, so extending the gills further forward on the adult should help compensate for the extra oxygen demand of the body.
Similar differences in the length of the gill line was observed between other adults and juveniles in a small sample, but a statistically valid study was not made. Leaflet size may be affected by micro habitat; those in holdfasts with less aerated water tend to have bigger leaflets than those on fronds in open water (Graham & Fretter, 1947).

Concise
Part 1, SHELL FORMS: 02 Patella pellucida. Growth Stage 2 (GS2). July 2012. Littoral. North Yorkshire, England. © P. Lightfoot.
Part 2, BODY & ANATOMY: 03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.
Part, 3 HABITS & ECOLOGY: 04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.