I beg to differ. I have lots of experience owning and buying leather and denim jackets and always size down one and even two whole sizes. You are completly correct in your assumption that your leather should have a tight fit. There's nothing wide than owning baggy leather ( have you seen leather jackets that add a gut? Not appealing).
Just the .02 cents of a biker.
Indeed. As a fellow biker, I can personally vouch for the need for a jacket to fit closely.
There is the practical issue with a bike jacket of not wanting clothing that catches air and drags and flaps in the wind: noisy, uncomfortable, and even slows you down. And of course, a bike jacket is protective clothing: not much good if it rides up under your armpits leaving your body exposed if you have a low-side. That's why real bike jackets are normally tighter, and rather stiff and/or reinforced. "Fashion" bike jackets, not.
If you mean about leather quality, there is no such thing as "Topman leather", as they don't make anything themselves. Just assume it's very cheap. If you mean about fit, then as we're talking about a cheap fashion garment rather than an actual bike jacket, the sizing is hard to guess. But my guess is that a jacket, even made of leather, aimed at young men with limited budgets, is going to fit most slender teenaged types pretty true to size, and already be close-fitting by design. Also, even if the leather stretches, the polyester lining won't. I suspect too small is too small. But maybe if you enquire they can give you a definition of what "small" and "extra small" mean in their sizings, with chest, waist and shoulder measurements. You can be certain they have a standard, and it might be different to another brand you're used to.