Yes Quinoa is fattening just like rice and actually Quinoa is a bit more fattening than rice is.
Quinoa has higher amounts of protein and lower carbohydrates per serving than rice does.
So if you're looking to fatten up and gain weight then you would gain more weight faster eating Quinoa.
If you're looking to lose weight then eating rice would be the better option since rice contains less amounts of protein and carbohydrates per serving.
Rice is the better option to eat when on a diet and losing weight but Quinoa is still okay to eat when losing weight but I would just avoid eating too much Quinoa if weight loss is your goal.
Eat the Quinoa if you need to gain weight but both Quinoa and Rice are healthy foods to eat.