I emailed Westone, JH, and UE last night to ask them your question, posing as you. The only company to email me back by 2 pm today was Westone. Great job on customer service!
Here's their reply:
Thank you for your email. You are correct in saying that gaining or losing weight could change the size of your ears slightly. Each person is different. We would recommend that you hold off until your reach your desired weight loss goal, as this could effect the fit. Please let me know if you have any questions or concerns. Have a good day!