Actually, I would like to offer a different answer.
First, if depends of the MRI is open or regular standard kind. Regular MRI's all have a weight restriction based on what the table can hold, and the odds of a larger person being able to fit through the scanner. It depends upon the machine brand and how old it is. Every closed MRI has a different weight specification. Any MRI facility will be happy to tell you. The typical limit for table weight is 350lbs.
An OPEN MRI (as in the above link in the previous post) usually do not have such limitations.
Here is a brochure regarding the very subject which discusses this in depth.
Please let me know if you have any follow up questions for clarity.