Have you ever found a verse of Scripture that says, "The thin will inherit the earth" or "Blessed are those within the target weight on the chart?" Are the thin and fit people blessed by God because they are thin or fit? Does God even care?
Do you believe that weight loss is an activity that is blessed by God? Does His favor rest upon you more when you deny yourself that Snickers bar or take that extra lap around the track at the gym?
If these things were true, what would that say about God? Think about it...
If your motivation for wanting to be thin or lose weight is to "please God" then I would say your motives are honorable, but you are missing one crucial point: if you are a born again Christian, the favor of God, the love of God, acceptance by God, the pleasure of God already rests upon you. God is already pleased with you exactly as you are, right now. You cannot do one single thing to undo what God has done for you in His Son Jesus Christ.
His pleasure rests upon your because of who you are in Christ! The righteousness that is yours in Christ and the Spirit that indwells each regenerated Believer is what makes you acceptable before the Father. To think that there is some action or anything else that you do makes you lovable, acceptable, or suitable to God is to place yourself in the position that belongs only to Jesus Christ. When you do so you are discounting what He did on the cross, believing and acting as though the cross is somehow lacking in sufficiency and needs my help for atonement. I must make up by being good what Jesus lacked in His sacrificial death.
Are we to obey- yes! Are we sinning when we don't obey? Yes. Is God displeased with my sin? Yes, God hates sin! That is why only the sacrifice of God in human flesh would satisfy our sin debt. The risen Savior Jesus stands as Advocate before the Father and when God looks at you and me in Christ, all He sees is Jesus the perfect sinless one!
The Father does not see our sin, for all our past, present, and future sin was forgiven at the cross. God has exacted His pound of flesh from His own Son. It is important you understand this, because if your reasoning for being thin or not overeating, or if you are trying to hide yourself under layers of fat from overeating thinking you are punishing yourself to gain God's favor you can stop right now- you already have all the favor from God you will ever have or ever need. Because of grace, because of Christ.
Furthermore, because of Christ you have the ability to change your sinful behavior of all kinds not just your dis-ordered eating behaviors. You have been enabled to change in ALL areas of life.
seeing that His divine power has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence. 2 Peter 1:3 (NASB)