Albert Barnes on Hebrews 13:22

Suffer the word of exhortation. Referring to the arguments and counsels in this whole epistle--which is, in fact, a practical exhortation to perseverance in adhering to the Christian religion amidst all the temptations which existed to apostasy.

For I have written a letter unto you in few words. This does not mean that this epistle is short compared with the others that the author had written, for most of the epistles of Paul are shorter than this. But it means, that it was brief compared with the importance and difficulty of the subjects of which he had treated. The topics introduced would have allowed a much more extended discussion, but in handing them he had made use of as few words as possible. No one can deny this who considers the sententious manner of this epistle. As an illustration of this, perhaps we may remark, that it is easy to expand the thoughts of this epistle into ample volumes of exposition, and that in fact it is difficult to give an explanation of it without a commentary that shall greatly surpass in extent the text. None can doubt, also, that the author of this epistle could have himself greatly expanded the thoughts and the illustrations if he had chosen. It is with reference to such considerations, probably, that he says that the epistle was brief.