It depends on whether you want to be a military or civilian pilot.
For military, then the answer is yes, you do need to be fit enough to pass the service entry requirements. Remember that fitness is independent of weight. Best speak to a recruiting officer of the particular service you are interested in to find out the exact requirements, or check the relevant websites.
For civilan flying the requirements are obviously less stringent (since you're generally not dealing with high g environments or getting dropped behind enemy lines). As said, weight is less of an issue, and is taken into consideration with other risk factors. So if you have a high BMI and are diabetic then questions may be asked, but it does not necessarily mean that you wont get the medical. Best place to seek advice is a friendly local AME or give the medical unit a call at the CAA.