A correspondent from the Czech Republic, having noticed that the amount of space devoted to individual letters in a Czech dictionary is different from what is found in English, writes to ask why some letters of an alphabet contain more words than others.
It's an interesting question, which it's possible to answer in general terms but not always in detail. The basic point is that writing systems reflect phonologies, and the individual vowels and consonants of a sound system occur at different frequencies (there's a list of the frequencies for English in my Cambridge Encyclopedia of the English Language). So, when an alphabetic orthography is first developed, the letter frequencies are likely to reflect the sounds. That is why, for example, in English the 'largest' letter in the dictionary is /s/ - this is because English phonology allows more clusters of consonants (C) in which /s/ is the first element than for any other sound - sing with C-, sting with CC-, and string with CCC-, this last sequence being the most complex you can have in initial syllable position in English.
The complication is that, over time, other factors intervene, so that the original phonological system becomes obscured. In English the arrival of the Normans meant that French spellings complicated the originally clear relationship between sounds and letters. Classical spellings caused further complications in the 16th century. And it only takes an influx of loan words to alter the balance of letters in a language or even to introduce a letter which was not there before. There is no native w in many languages, for example (such as Spanish), but loan words (such as World Wide Web) have meant that there is now a W section in a modern Spanish dictionary.