I have a gap between my two lower end molars, had crown put in on second last molar but it was made bigger to cover the gap?

The issue is how big of a space had to be closed up in order to extend the crown. Sometimes, the best course of action is to split the difference between the 2 teeth, and widen both sides slightly to close the space. Unfortunately, without a photo or x-ray it's hard to determine the answer properly. If the space wasn't too wide, extending the crown slightly is not a bad thing.

Your dentist made the crown wider in order to close the space between your teeth as this will prevent food impaction between your teeth. In order to prevent decay under the crown, be sure to floss diligently.

In short, it should not be a problem.  In most cases like this the laboratory or the the doctor designing the crown is able to get the desired margins, line angles and contacts to prevent issues like recurrent decay around the crown.  The reason they probably did this was to maker sure that they close the spaces up to prevent food or debris from being trapped between the teeth, which ideally should lower the chance of tooth decay.  You should be getting a nice snap with the floss between the new crown on both ends and generally as long as you are brushing, flossing, and taking care of it it should not be an issue.  From time to time it is something that  I will do in my practice as well but only if the situation would benefit the patient.  I would guess without pictures or x-rays that you fit nicely inside the realm of that type of treatment.  If you do have any further concerns I would follow up with your dentist, but truly I think it will be fine.