I am no 100% certain but think that 1 lb. Is equal to one length so, although there is no maximum weight no owner or trainer would want a jockey that is several pounds overweight to ride their horse in a race.
I am not sure of the weights for racing in the USA but in the UK the jockeys that get to big in height and weight have the opportunity to become steeple chase jockeys. The heaviest weights for jump races can be 175 lbs. This is because the horses are older and the races not so fast as they are over longer distances.
Most jockeys spend their lives wasting, keeping the weight to a minimal and often several pounds under what is should be. They spend a lot of time in saunas, running and generally keeping as fit as possible. Some are naturally small and light but generally most keep their weight down as light as possible so they can ride the lighter handicapped horses. They get paid by the rides they get so, of they are a couple of pounds weight over they will loose the ride.
There are two things that get jockeys 'down' in their career, in the UK, first is the wasting and second is the travelling as trainers all own their own yards which are never at the race courses.
A typical day for a jump jockey would be to ride out at around 7 a.m. Breakfast, usually black coffee and maybe a piece of toast. Depending where they are racing that afternoon they might ride out or school another horse (schooling is jumping it over fences) travel to the meeting, then home after the racing has finished.
Travel in the UK might not consist of the distances travelled in the US but the roads are congested so plenty of time has to be allowed to get there.
Considering most jump jockeys expect to fall once in every six rides, life is tough as they are not allowed to ride until a doctor has passed them as fit.
I reckon that of all sports race riding is only second to bull riding where those lads know they are going to come off every time they ride, even if they make the 8 seconds!
Flat racing is considered a lot safer as falls are less frequent but when they do happen it can be worse because of the extra speed they are travelling.