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applying any oil for 15 minute massage ,,only 10% of oil get absorbed by muscles,,so yes it is true it will help you to gain weight but it will take time...
Right kind of exercise can cause hypertrophy of the gluteal muscles i.e. make your buttocks bigger. Topical application of oils, etc. can help in preventing the stretch marks that may arise due to the sudden stretching of the skin. But it plays no role in increasing the size of the tissues.