Adjectives that are shorter than 3 syllables, are normally used with "er".
big - bigger
short - shorter
Adjectives that are longer than 3 syllables, are normally used with "more".
difficult - more difficult
comprehensible - more comprehensible
However, there may be exceptions with the short adjectives, which are sometimes used with "more" and not "er". I can't think of an example at the moment.
Also, there are exceptions such as "good - better - the best"; "bad - worse - the worst"