I practice NO-GI.
I very rarely get to spar some one who is not 20+ pounds heavier then me. I am usually able to achieve dominant position such as side mount or mount, but sadly cant hold on to it for very long facing these big guys.
They tend to just roll be over even tho I am almost sprawling holding on to their neck and hand. lol

Same with Mount I break their grips but their bridge is so explosive against me being so light that I eventually get bucked off.
The only time they can get rid of me is when I get their back. Then I am there like glue trapping their arms with my legs. lol. sweet revenge!
Can any one provide some tips for holding down big guys in Mount and Side Mount?