I would understand 'thick'. 'Hefty' refers to the bulk of an object, so the suggestion is that they have a lot of pages. Possibly the writer chose this word to avoid 'heavy' or 'weighty', which might be taken as a comment on the content.

Without the context of your quote it is difficult to be certain but it could either mean the magazines are large and thick or that the writer thinks they have some importance or value. It is an odd use of the word but not necessarily incorrect.