Background: I have a friend who is fat but confident and happy with her body, and below are what I have found helped myself and others understand that she is happy and has no desire to change.
Overall, the best way for you to convey your comfort with your body image is by your tone and the language you use.
My first recommendation is to avoid the word 'fat'. As confident as you are with owning this word as a descriptor for yourself with no negativity attached, society has used the word fat as a way to shame people. When your friends hear this word, their first reaction is likely to be "But you're not fat!" because it may make them feel like they're doing you injustice or hurting your feelings if they were to agree with you (or not rebuttal).
Instead of saying:
"I'm too fat for these jeans anymore."
Try finding a sentence that says the same thing, minus the word 'fat', such as:
"I don't fit into these jeans anymore."
"I don't fit into these jeans since I gained some weight."
My second recommendation would be to not skip a beat after these statements. If you say "I'm too fat to do X" and pause, not only are your friends potentially feeling the need to rebuttal you due to the reason I mentioned above, but it's likely if there is a pause in the conversation that they may also feel obligated to fill in the silence with a response. For example, instead of just one of the sentences above, you could try saying:
"I don't fit into these jeans anymore, but I'm planning on grabbing some new ones here soon!"
This gives your friends an alternative path to continue the conversation, without leaving them in a position that may be awkward or where they feel they need to justify your weight to you.
My third and final recommendation is that if you are confident, always act it. Since weight tends to be a touchy subject in our culture, don't just say you're comfortable - live by it. Use positive, confident body language when you're talking about your weight to let people know you're comfortable. The above examples have focused on how to phrase the limitations of weighing more, but don't forget to admire yourself! Look for ways to compliment yourself for how you look and voice those when they come up, such as:
"I love these jeans, they compliment me well, what do you think?"
It's my opinion that doing the above will help speed up this process of getting your friends as comfortable as you are, but the most important action is to always continue to love and own yourself and your body. Your friends will catch on with time, regardless of what steps you take.