Hop Addition Calculation

I am working on Citra MO SMaSH using Beer Calculus.

10lb Marris Otter
Citra Pellets
1056 Yeast

I put it in with the boil at 3 gal for 60 minutes and ended up with hop additions:
.5oz@60
.25oz@45
.25oz@30
.5oz@5
.5oz Dry
And got 32 IBU.

I changed it to 6.5 gal for full boil and it jumped to nearly 60 IBU
I had to amend the additions to:
.5oz@60
.75oz@5
.5oz Dry to get it back down to about 32IBU.

Is there really such a dramatic difference in utilization?
Which way should I go create a light balanced Pale Ale?

Move more of the hops later. Go for a single addition at 60 for whatever amount gives you about 30 IBU's and then throw the rest in there between 15 minutes and flame out, not counting any you want to dry hop with.

Thanks. The MO is mashing right now.