It's not a particularly large or bulky extension. It's pretty standard in context of the existing.
Just from the side and rear elevation it's clear that the extension is subservient - the large existing gable, parapet and high existing roof help minimise the appearance of the extension. It's not even borderline big.
The reference to the ridge length is nonsense, as is the reference to breaking up the "uniformity" of the eaves.
It seems like the planner has taken an exception to the proposal, for whatever reason, and then tried to justify refusal with some tenuous statements.
Design detailing could be better. It's not helped the proposal, but perhaps there was no inkling that the proposal would be rejected, and so the designer has not paid better attention to design detailing and presentation. Normally, if a proposal needs a little push and planners persuaded, the designer (a good designer) would concentrate more on detailing and presentation and include extra views to "assist" the planner in reaching the right decision. Emphasising and deemphasising things as necessary.
As it stands, I would suggest it is appealed.
At the same time, you should explore what alterations are needed for a resubmission.
Obviously there are costs for both these, unless you do it yourself. Apart from your designer being a bit crap, I don't don't think you can blame them. OK, perhaps if they replicated the parapet and corbel features in some way, and had given more thought to the windows it may have looked better and been more persuasive - but as we don't know their brief, it's difficult to judge.