OK...There are MANY factors to consider. Not all "skinnies" and "fatties" are created equal.
I am often approached by women in the gym who say, "You look great, how can I look more like you?" And the truth is, if the woman is kind of thick (she does train but needs guidance with proper diet) SHE WILL HAVE A MUCH EASIER TIME GAINING LEAN MASS than her "I am starving myself into a negative clothing size" counterpart. Why? Because the thick chick IS NOT AFRAID TO FEED HER BODY so with the proper guidance she can transform her body fairly quickly (six months - not six days LOL). Whereas the skinny chick who has barely the strength to walk upright but Goddamnedit - she is a size 2!!! (like is so great when she needs a nap to make it through the midmorning because her brain is so starved for food) could lift until she literally pass out! Her metabolism WILL NOT SPEED UP because not only is she not feeding her body enough to MAINTAIN her mass, but she is actually USING THE MASS SHE HAS to survive.....SLOWER METABOLISM - eventually she will either GET REAL FAT (when she begins to eat...we all have to start eating some time).
It should not be about WEIGHT LOSS, but about gaining lean mass to speed your metabolism so you can burn calories more efficiently and have a better quality of life.