Is obviously fairly arbitrary except for the purpose of election expenses, but I was still surprised when browsing the map at just how arbitrary the divide seems to be. Here's a list of the largest borough constituencies by area, and the smallest county constituencies by area, by new boundaries. England only because I couldn't find area data for elsewhere.
I have to think it would make more sense to just specify a dividing line- e.g. county if over 65 km^2- because I really can't think of any logical criteria which would lead to the constituencies listed being sorted into county/borough status in the way they have been.
County constituencies under 70 km^2Maidstone and Malling 51.89 km^2
Chatham and Aylesford 56.27
Hemel Hempstead 57.41
East Worthing and Shoreham 57.48
Leeds East 58.12
St Albans 59.72
Wigan 60.55
Broxtowe 60.77
Hazel Grove 61.43
Stalybridge and Hyde 63.28
Stevenage 63.45
Gedling 64.01
Dartford 65.41
Rayleigh and Wickford 68.68
Houghton and Sunderland South 69.20
Borough constituencies over 60 km^2Redcar 82.68 km^2
Basingstoke 78.67
Castle Point 76.87
Thurrock 71.35
Washington and Gateshead South 69.85
Orpington 67.89
Milton Keynes Central 65.83
Lincoln 63.97
Woking 63.64
Spen Valley 62.15
Basildon and Billericay 61.88
Halifax 61.54
Wakefield and Rothwell 60.71