Seriously...if you feel uncomfortable wearing jerseys at your current weight and/or level of fitness...save your money and don't wear them or wear a t-shirt over your current ones; and work at it until you do feel comfortable wearing them.
If weight and fitness level is not the issue...there are plenty of resources on the internet and relatively inexpensive equipment available in order for you to tighten up your chest.
My point? Don't let "cyclist peer pressure" make you do something you don't feel comfortable with.
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Elwood: It's 106 miles to Chicago, we got a full tank of gas, 1/2 a pack of cigarettes, it's dark and we're wearing sunglasses.
Jake: Hit it.