Hello, I Am Prescribed 70 Mgs Of Vyvanse A Day,

Hello, I am prescribed 70 mgs of vyvanse a day, I see everywhere where that is the max dosage, one 70 a day honestly doesn't do anything for me, and it lasts maybe 2-3 hrs. I am also 6'5" 265lbs, and have built a tolerance over the years. Would it be smart to ask for a higher dosage and is it legal to get one? I have taken about 4 a day before and on average 3.