Yes it is too small (presumably the grass is not part of their enclosure). The sad thing is the mighty (and financially well off) San Diego Zoo has a relatively new lion exhibit that is about the same size!
The zoo is probably financial capable of building an new lion exhibit, the problem is that there is no space in the park and this exhibit is a listed building.
@Arizona Docent a Hagenbeck original, moat and all. (Hopefully they can find some way around the "listed" status that pervades many woefully inadequate exhibits and expand the space, maybe giving the lions access to the roof of their holding area along with planted grass...?)