Is this really about the pressure to bulk up, or is this about wanting an ally in your family? If that's the case, it's fine, but you should recognize what's driving you right now.
Your brother sounds like he cares, but how close are you really? What are his views on LGBT issues? Can you actually picture him coming to calm acceptance, or does that not seem to be in the cards? If you really believe that he will be accepting and that he might be supportive, then go for it. Maybe test the waters first, though.
If you already have a plan for coming out, don't let this rush your plan. Do you have a plan, may I ask? If so, where does your brother currently fit into it?
If you just need him to lay off, try to be more clear. Older brothers, in my personal experience, can be notoriously thick. If he thinks he's helping, he clearly has not gotten the message that his "help" is unwanted and is even starting to hurt. Maybe that's the way to approach it- let him know that it's hurting you. If he truly wants to help, then this might be what he needs to hear. Take him aside, tell him that he's hurting you, and you need him to stop. If he really cares, then although he might be confused by this, he'll also hopefully give his "coaching" a rest.
Good luck, girl. Stay strong.
-Teg