|To stop/conceal muffin top inside of low rise jeans or swim suits you can create a casing on the inside. You can stitch in the ditch or hand sew using a pick stitch. In jeans the top of the casing would go right under the waist band, which should be slightly widened.
The casing carries a flat thin belt,it can be made from twill tape with a flat metal pant hook set for the closure sewn on, or button elastic with a button on one end, or just tie ends which are left longer. This will act as your real closure, on jeans you would still sew a zip, and use a button outside for looks.
What is important about the casing is that it goes a few inches below the top edge of the garment. You will cut whatever is above the casing with zero ease accounting for outside closures fastened (but no sucking in).
This lets the cased inside closing come in tight enough to hold up the garment, then the fabric above the casing rides smoothly up the sides of your muffin top. The eye will go to the top edge of a skirt, pant, or swim bottom.
From the bottom of the casing down your garment can still be as tight as you like. It is only above the casing that pattern changes may be needed to widen to zero ease for smooth fit and comfort. If any of this is unclear let me know as I will check back to edit if needed. I hope this helps esp. for the swimsuit contest, have fun.