'The rather ... more abundantly.' This is a common Greek idiom, but it is impossible to render it exactly in English. The A.V. translates the expression 'exceedingly the more,' but in English 'the more' supposes more than something, and because of something. I do not think it always merely emphatic; there is an unexpressed motive which is the cause of 'the rather' in the mind. I suspect that ver. 14 gives the clue to the force of it.
It may, perhaps, be translated 'and in (or 'besides') our encouragement we the rather rejoiced more abundantly in the joy of Titus (for his spirit is refreshed by you all), because if I,' &c. There are often examples of the change of 'I' and 'we' in this epistle; it is the case in this very context. But not without a reason in the sense, 'I' being more personal to Paul.