Someone who has been heavy all along, I'd say, would have a more difficult time. It has a lot to do with how you already see yourself and what you think is acceptable for yourself. Someone who has battled weight issues all their life identify (even if negatively so) with their weight, which can keep the struggle going.
i think the latter, mentally and emotionally they most likely will not have a good opinion of how they look, may have low self esteem and low confidence, and i think would be likely to have less motivation to get the weight off whereas for someone who is used to be slim, i imagine they would find it easier to motivate themselves into losing the weight and would find it easier being self discplined simply because if they are used to being slim, they would be more determined to get back to their normal weight. Thats what i reckon anyway.