When you order a pizza... What would be our reaction if you are delivered 100 pizzas just because there were room in the car?
That's the same idea: someone needs 3 tons of coffee. He doesn't need 300, just 3. The fact that you are using a T7 or a sidewinder is not his problem.
On the other hand, having a T7 enables you to accept the few missions where plenty of tons are required. And delivering 3 tons of coffee enables you to deliver also plenty of other things the neighbors may need.
Typically, I'm working on my alliance reputation. I have selected 4 Alliance-affiliated stations not far one from the other, in which I can find almost all the commodities all together. It enables me to accept ALL the "We need X tons of Y" missions, I know that I'll find some Y in one of the 4 stations part of my cycle. So, I repeat the cycle again and again. And since my cargo is never full only with these missions, I purchase every time the best-profit commodity I can deliver in the next station of my cycle to fill my cargo. It works perfectly with my T6 - with a T7, I'll be able to purchase much more "fill cargo" commodities, while still being able to complete all "need Y" missions. As a trader, you don't need missions to earn money. In fact, you'll earn more money doing pure trade than doing missions!