Increasing PGA size not improve hit ratio

Hello,

I got a Oracle 10.2.0.5 database under HPUX.

It has a low PGA hit ratio, around 60%, but the advisors not recomend increase the PGA size because it not improve the hit ratio.

PGA Target Est (MB)	Size Factr	W/A MB Processed	Estd Extra W/A MB Read/ Written to Disk	Estd PGA Cache Hit %	Estd PGA Overalloc Count
256	0.13	20,266,037.85	20,747,058.82	49.00	21,918
512	0.25	20,266,037.85	13,044,570.02	61.00	0
1,024	0.50	20,266,037.85	13,024,742.19	61.00	0
1,536	0.75	20,266,037.85	13,024,742.19	61.00	0
2,048	1.00	20,266,037.85	13,011,638.17	61.00	0
2,458	1.20	20,266,037.85	12,995,329.55	61.00	0
2,867	1.40	20,266,037.85	12,995,329.55	61.00	0
3,277	1.60	20,266,037.85	12,995,329.55	61.00	0
3,686	1.80	20,266,037.85	12,995,329.55	61.00	0
4,096	2.00	20,266,037.85	12,995,329.55	61.00	0
6,144	3.00	20,266,037.85	12,995,329.55	61.00	0
8,192	4.00	20,266,037.85	12,995,329.55	61.00	0
12,288	6.00	20,266,037.85	12,995,329.55	61.00	0
16,384	8.00	20,266,037.85	12,995,329.55	61.00	0

How can I then improve the PGA hit ratio? Is there a reason to not improve it increasing the PGA size?

Thank you guys