Yes a canary can get too fat.
Fat can be seen on the breast and abdomen as thick and yellow under the skin.
The fat is usually caused by having a bird in a small cage, where it cannot get enough exercise, or by feeding a diet with too many oily seeds, particularly Rape, Linseed and Maw seeds. Also a diet high in Egg & Biscuit all year round can create too much fat.
When a bird is too fat it shortens its life and reduces the conversion rate when breeding.
To reduce the birdsí fat, increase the amount of greens and fruit and reduce the oily seeds and tit-bits. Feed plain canary and millet.
To stop obesity occurring in the first place feed a variety of foods, with about 20-25% of the diet being greens/weeds/fruit.