Im not a planning expert but the following might help.
Back gardens are a sore point t. I have a side drive biig enough to get a fire engine downbut it would not be allowed. The main reason would be over development. Local feeling does not really come into it. If the application does not comply with planning bylaws then it should be turned down. Its not enough to say 'Its too big'. How big is big? Most of us are layment and rely on planning departments to do this job for us. However, the more objections the more likely it is to be rejected. I have always found planning officers very helpful. Give them a ring and ask them if they are going to support it (or not). Then ask them what reasons you can use in your objection. Its their job to advise you. Ask them about (1) the 45 degree rule (2) density (3) overbearing on adjacent properties (4) visual impact on street (5) are there any trees which could be made a subject of a preservation order (6) access for emergency vehicles (6) building line.
Unfortunately you have to show your name and address on the objection.