if the larger animal is showing higher lwptin levels at proportional fat levels then it can be possibly because of leptin resistene causing hypertrophy and hyperplasia of adipocytes and relative leptin resistence  in contrast to the smaller one with smaller fat mas relative to body composition.

Thank you for answer, Kulvinder. 

In my explicit case, the larger mouse is showing increased lean mass, normal fat mass, but increased leptin levels.

Leptin levels appear to correlate with total body fat and mass. Leptin has even been shown to be secreted by muscle issue(pretty cool). Thus, since muscle serves as axillary tissue for Leptin production(notably, at differential rates), a higher fat body composition may lead to distinct Leptin levels even though overall bodyweight remains the same. 

Well, thanks a lot, Michael. I absolutely didn't consider the contribution of muscle mass to leptin production.Do you have a particularly interesting paper for me on this subject?

Laura,

Here is a paper that shows that muscle mass contributes to whole body leptin production. 

http://www.sciencedirect.com/science/article/pii/S104346661200662X

Thank you!

furher anotherthing that comes to my mind is what is the status of long and short leptin receptor although it is not a ob/ob or db/db mice  but which mice yo have used to study as alli can see is rodents have you seen their receptor levels although leptin levels are high .

How long were the mice fasted? Were they male or female? What were there insulin levels? Although you are asking about bigger and smaller rodent of same species, strain of mouse can make a big difference to metabolic parameters. 

not only that did you study the central pomc.npy,agrp,cart levels in these mice besides the elevated leptin levels recently angiotensin 1receptor in subfornical region has been found to mediate the effect of leptin on energy expenditure in BAT although having no effects on allthese other neuropeptides since angiotensin converting enzyme inibitors are associated with various changesin pbesity with telemesartan having different effects so you should see the effects on central neuropeptides as well .