To think thin belongs to the class of idioms that are formed by the verb to think plus an adjective (Think big (about your future, a big expensive vacation), think small (anything really that is distinctively small), think pink (for example when decorating a nursery for a newborn baby girl), etc.). While the translation will always be different depending on the context, the general direction is ориентироваться на, верить в, думать в определенном направлении, концетрироваться на определенном аспекте. Think thin is a popular slogan for low fat food, flat screen monitors and TVs, etc.
Your character could have said
Я, знаете ли, на диете или Я только о том и думаю, как похудеть
Or she might as well be shopping for a new TV and is trying to convince her husband that the flat screen is the one they have to have:
Я думаю, чем тоньше тем лучше.