I have a full size Gray mat and found it comfortable, but I am a little overweight. The mat bent and gained a shape of my body after several procedures. Is it fine or I need a firmer basement?

For better effects and higher durability, it is always recommended to use the mats on a more stable basement, such as massage tables. 

But the mats work fine, and they acquire the shape of the body if you use them on the standard mattress. Many customers sleep on the large mats, and it is completely possible no matter what the weight is. 

The strengthened heating system is very durable, and there should not be any issues if the mat is bent. You will even feel more comfortable if the mat is shaped for your body.