The only way SIN will let a larger bottle of liquid through is with a letter from your doctor/optometrist explaining that the liquid has been prescribed to you and cannot be obtained easily in Asia. Even then I would expect a supervisor to be called and for this to take a while. I am aware of a colleague who recently did this with medication in a large bottle, but it was quite clearly labelled by the pharmacy with my name etc, and I had a not issued by a doctor based in Singapore. Even then it took over 20 minutes to get through security.

As someone else above pointed out, SIN security can be inconsistent and let things through on one day but not on another.