Is it immoral to be fat?

No sir, Obesity does not protect from cold temperature** nor it acts as a perfect insulation**. Sensitivity to cold is in the skin not beneath the skin. Obese people still have to wear warm clothes in cold weather just as non-obese people. No measurable difference in sensitivity to cold.

Many people who live in cold areas do eat more high calorie food to create more energy to keep them warm. Also they wear thicker clothes and hence they do not really care for or aware of becoming obese. Compare someone who lives in warmer place at beach-side (who wears swim-suit regularly) to one who lives in very cold places close to northern hemisphere.

Very Obese people may feel warm just because of skin touching/rubbing with skin and hence they develop sweat in those areas and many develop skin infection with fungus (candida).:(

Catching a cold also not the same as acquiring obesity. One may not have control on catching cold but one has perfect control of not getting obese. People don't go around and deliberately catch a cold. Infected people are more responsible in giving someone cold than the person receiving it.

Only thing one can think of obesity as positive that it protects from dying from lack of food/starvation but at the expense of by-products of fat converting in to sugar i.e. ketones/beta-hydroxybutyric acid and acetones etc.

Like I said earlier, one can find a lot of negatives with being obese and very few and indeed arguable positives.

Coming to isolating obesity and calling it not being immoral is like saying urea within urine (an essential component of urine) itself is not bad but urine is!!! Hope you got it.

OR

If isolated fat cell from pork meat while still attached to meat is chemically same as isolated fat cell from halal beef so one can say, pork fat cell is not bad?

Isolating obesity from its cause being deliberate over-eating or its results of acquiring more diseases is a microscopic argument. I don't know what others call it but I could only come up with this term.

What I mean you are trying to isolate obesity from all the bad things which come along with it, before or after. So nothing about cart or horse example there.

Re: Is it immoral to be fat?

depends....if you consider being over weight as a sign of lazyness, greediness etc. or simply due to slow metabolism. the latter is probably just an illness.