Our own head is a special part of our body. First of all it is not something we can look at, except indirectly through a mirror or a photograph. It is also likely to be something closely linked to our sense of identity.
Information about ourselves is often gathered from mirrors. This is one example (there are many others) in which van Gogh is right handed in the portrait by Gauguin (1888), but left handed in the self-portrait on the right (1888). This is probably a consequence of the use of a mirror
It is unlikely that the fact that we use a mirror to see ourselves, in itself, should distort shape or size. In general objects are correctly perceived within planar mirrors
When asked to estimate their own head size, people tend to overestimate it, relative to when they estimate someone else's head size. We know this from a series of experiments in the lab.
To integrate the experimental evidence with evidence from a survey of existing pictorial representations, we have compared the size of the depicted head in two samples: self-portraits (115) and portraits (124). Will there be a size difference in the paintings, depending on whether the subject is the painter's own head as opposed to a different person's head?.
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The first scattergram showing that there is a clear tendency to paint one own's head larger than the head of another person. This is true independently of canvas size, though in general heads do tend to get a bit bigger on larger canvases. The second scattergram shows that the trend is fairly consistent over the centuries.
Each cross is a self-portrait and each circle is a portrait. The line is a "loess" curve. Basically this is a bit like a running mean.
Although we analysed a large set of paintings, often we had more than one painting from a given painter. We selected a set of pairs, so that each pair included a self-portrait and a portrait by the same author. The overestimation of head size for self-portrait relative to portrait was again confirmed in this second analysis
A self-portrait (left) and a portrait (right) by Eugene Delacroix (1798 -1863), arguably the most important of the French Romantic painters.
More examples are provided here
References:Bianchi, I. Savardi, U. & Bertamini, M. (2008). Estimation and representation of head size (People overestimate their own head, evidence starting from the 15th century). British Journal of Psychology, 99, 513-531. DOI:10.1348/000712608X304469