Obesity is a medical condition in which excess body fat has accumulated to an extent that it may have a negative effect on health. People are generally considered obese when their body mass index (BMI), a measurement obtained by dividing a person's weight by the square of the person's height, is over 30 kg/m2; the range 25–30 kg/m2 is defined as overweight. Some East Asian countries use lower values.
Pious persons who are the practicing believers will be in Paradise as young and nice looking, ( You will recognize in their faces the brightness of delight) Sura 83,Verse 24 ( تعرف في وجوههم نضرة النعيم ).
But those who insisted on committing major sins and wrong faith and or wrong acts will be looking ugly, though they will be known by those who knew them in this life ( They recognize each other
( يتعارفون بينهم)Sura Younis, Verse 45.
Allah (SWT) says in Quran about The Day of Judgement ( On that day, those who disbelieved and disobeyed the Messenger will wish to be crushed with the earth, although, they will never be able to hide anything from Allah). (Sura 4, Verse 42) (يومئذ يودّ الذين كفروا وعصوا الرسول لو تسوى بهم الأرض ولا يكنمون الله حديثا
There is Supplication recommended when we perform Wudhu; O Allah, make face bright when faces ( of the disbelievers) turn ugly, and never turn my face ugly when the faces ( of the believer ) turn bright.
اللهم بيّض وحهي يوم تسودّ فيه الوجوه ولا تسوّد وجهي بؤس تبيضّ فيه الوجوه.
We will appear in the afterlife in a recognizable form such that those who knew us in this life will recognize us there.
However from narrations it can be deduced that those who go to heaven will be in a more idealized form (for instance, youthful or full of light) and those who commit severe sins may see those sins manifested in their physical form in their bodies.
(Perhaps this is similar to how when we see someone in real life whom we haven't seen for decades, we can sometimes still recognize them even though they look different; there is something about the essence of the person that we recognize rather than the purely physical appearance.)
Whether it is ideal or not to be fat is of course cultural since in some cultures it is/has been considered a mark of beauty/handsomeness and in other cultures it is not. But what can be said is that the way the people in Paradise will appear will come across as positive, regardless of the details of what the actual appearance is.
Having big belly can be genetically inherited and is not necessarily a result of over eating.
We do not have in Islam any criticism of men or women with big bellies, but we do have criticism of over eating. (Human being did not fill a vessel more harmful for him than his stomach) is just one example of Hadeeths in this regard.
We have Hadeeths encouraging eating less, speaking less, and sleeping less as a way of healthy life. Less here means avoiding over eating, over speaking, and over sleeping.