The difference between letters and full articles is described here [1]. I don't think that one should view the length of the paper or whether it's published as a Letter or an article as a marker for its scientific importance.

For instance, The paper that Watson & Crick published that described the structure of DNA was only slightly longer than a single page [2].

[1] http://www.nature.com/nature/authors/gta/2a_Manuscript_forma... [2] http://www.nature.com/nature/dna50/watsoncrick.pdf