You need an 'equine leg Vet' specialist. A race horse Vet or one that Vets for H/J barns is your best best.
Most of the knee enlargements that are hard are permanent. You need x-rays with the horse standing on it and with it flexed from front and side. It will find things like chips and will see arthritic changes. Since it is hard, it is probably not Bursitis.
Many horses are sound for years with big knees. Most eventually get to where they cannot completely straighten out the leg and show lameness.