does size matter?

From the summary of local authority performance 2012/13 I have inserted the top quartile performance and bottom quartile performance in against councils in size order and subtracted them.

Rank	District		Population		Top		Bottom		Difference
					Quartile		Quartile
1	Cardiff		345,400		5		7		-2
2	Swansea		239,000		3		5		-2
3	Rhondda Cynon Taf		234,400		4		7		-3
4	Carmarthenshire		184,000		4		3		1
5	Caerphilly		178,800		3		6		3
6	Flintshire		152,700		6		4		2
7	Newport		145,800		2		8		-6
8	Neath Port Talbot		139,900		4		5		-1
9	Bridgend		139,400		4		4		0
10	Wrexham		135,100		3		6		-3
11	Powys		133,100		8		4		4
12	Vale of Glamorgan		126,700		3		6		-3
13	Pembrokeshire		122,600		5		3		2
14	Gwynedd		121,500		8		1		7
15	Conwy		115,300		4		4		0
16	Denbighshire		93,900		11		1		10
17	Monmouthshire		91,500		8		3		5
18	Torfaen		91,200		2		7		-5
19	Ceredigion		75,300		5		7		-2
20	Anglesey		69,900		6		4		2
21	Blaenau Gwent		69,800		5		5		0
22	Merthyr		58,900		6		6		0