Do you know if you both have the same type of Anubias? There are lots of different kinds. Some can get quite large, and some can stay quite small.
The other thing is plants can be mysterious. Sometimes the same plant may grow great in one location in a tank, and not do very well in another location in the same tank. They can be finicky at times.
20 gallon, and yes we do have the same type (Anubias Nana), but his like 6" high and has many stems and leaves and is bright green and mines 3" high with barely any and yellow green