In fact, it is so large in relation to the drawing that perhaps this was part of a larger piece earlier that was later cropped out. When considering the size of his signatures in other artworks, it seems odd that he would have made it so large here in relation to the portrait of Christ.
During the Renaissance drawings would occassionally be cropped for a variety of reasons. The artist might be unhappy with one section and remove it as a result, or he might wish to pass around one piece to other members of his studio for them to study and follow a similar style themselves.
The iconic image of Christ has inspired many artists over the centuries and was particularly frequent at the time when religion was embedded in western society at the most, which was around the Renaissance and for several centuries afterwards. Whilst it still remains highly significant, there are many other competing forces now such as modern culture in the form of television, film and sport. Jesus Christ, a source of inspiration for millions of people around the world, features on several other occassions within Durer's career, including Crucifixion, Crucifixion (woodcut), Christ and the Disciples at Emmaus and Christ's Entry into Jerusalem.