The tip of a pen is made to be very small, sharp, thin and pointing. This is to make it penetrate some thing like a cloth, paper etc... When we want to stitch some thing, we need the needle to penetrate in to that material... so it is sharp.
The head is broad , so that we can hold the needle at the head. To have a good grip, the needle is broad.. also, the needle used for stitching needs to have a thread passing through it... there is a hole through it.. so the head is large to allow for the hole..