The idea that St. Thomas Aquinas was "portly" or even obese is likely an exaggeration that occurred over time due to overemphasis of certain aspects of his appearance. Early accounts depict him as being both very
tall and
strong having a
big head, often with a build closer to a wrestler or football player than that of an obese man. As far as I can tell, St. Thomas was certainly an imposing figure but people have decided to engage in exaggerations based on some accounts of his appearance to the detriment of others.
For example, one of the earliest works covering St. Thomas' life was written by William de Tocco in the early 14th century, St. Thomas is described as "showing himself a robust and virile man" during manual labor. Contrary to the extremely exaggerated accusations of extreme gluttony by people like Shibley, William de Tocco emphasizes that the physical stature of St. Thomas was in accord with moderate and virtuous conduct which would preclude severe gluttony, "[I]t seems that God had fashioned his body as the noblest of instruments, which St. Thomas always held subservient to acts of virtue and which he never permitted to contravene the judgement of reason."
The iconographic tradition is also not uniform, with large variation across the centuries. I'll link some early depictions of St. Thomas Aquinas from the 14th and 15th century that don't match the "morbidly obese" claims:
https://upload.wikimedia.org/wikipedia/commons/c/c2/Lippo_Me...
https://www.kressfoundation.org/kress-collection/artwork/498...
https://catholiceducation.org/en/culture/art/saint-thomas-aq...
https://catholicclassicalict.wordpress.com/wp-content/upload...
https://www.wikiart.org/en/fra-angelico/st-thomas-aquinas-14...
There is another source I recall reading recently that gave credence to the thinner depictions, but unfortunately I haven't been able to dig it up.