Sounds like you have higher tendon tricep inserts. So that full tricep sweep, starting from the elbow upwards, is not present. Can stretch it all you want, but those higher inserts are going to stay the same. That can also go for high biceps, calfs, etc. Some people believe that by doing "low" lat exercises they can develop a lower lat sweep. This, along with the fore mentioned triceps, is not true. Like everyone else, you came into this world with the genetic body you were given.
This is not to say that a full potential for size and strength is not possible. Lot of big guns out there who do not have that perfect structure of the triceps. Lot of big guns out there that have flat, rather than high peaked) biceps. Guy's here gave a lot of good suggestions for improved triceps exercises.
One thing I do not know is why working the legs will produce larger arms (I understand the truth that more hormones & growth chemicals are released into the blood stream from the glands for muscle building). For some it may be true, but for most I would disagree. See a lot of heavy squater's (and other leg exercises) either BB'er or PL'ers, who have relative average arms. Seen others who are upper body artist,never working the lower body, with outstanding arm development. One thing that does seem to help with impressive arm size is doing some serious delt work with arm work. Like overhead presses (BB or DB), PBN's & Up-right rows or Hi-pulls. Gymnast come to mind when speaking of outstanding and complete armdevelopment and very light legs. Good luck.