Just as fat handwriting reflects a particular personality type, so does thin handwriting have its own interpretation.
While fat writing un-selfconsciously takes up space and sprawls across the page, thin writing is very contained, and restrained. In appearance it tends to be lean, cramped or even constricted.
Thin handwriting is often tall but in general it seems to be rather reluctant or hesitant about getting to the other side of the page.
Sometimes lacking in spontaneity, writers with thin handwriting seem to dislike showiness and prefer to keep out of the limelight.They are cautious and circumspect and tend to be reserved and self-conscious. They are not always the most generous of people but tend to be frugal and conserve what they have.
Of course the picture changes hugely when thin writing is combined with other aspects such as slant, depth, size etc. Under these conditions thin writing takes on a completely different meaning.
For example if the handwriting is both tall and thin and vertical, it could mean that the writer is a scientific thinker or someone with high aspirations. To understand the permutations you would have to know more about the real hidden meaning of handwriting. Check it out if this interests you!
Here is a sample of thin handwriting of someone with a scientific approach:
By way of contrast you may want to take a look at this article about fat handwriting