Most that are new to bonsai do not realize that the vast majority of "Finished, Exhibition Bonsai Trees" spend a good portion of their development as a tree that was 5 to 20 times taller and or wider than the "finished design". First the seedling is grown up and out, until the trunk is of sufficient diameter to support a design. Then the tree is reduced in height to bring everything back in proportion. This also happens with junipers. Most exhibition bonsai are at least 30 to 60 years old. Usually the first 15 to 20 years the tree is allowed to get large, to beef up trunk diameter. Only after the trunk is sufficiently large, it is then reduced in size. Then the next 10 to 15 years the tree is trained to conform to the idealized style.
You can size up junipers in containers. But it will take time. I have a juniper that started out about the same size as yours, It is over 15 years in my care, it is now finally ready to style. If was cut back a few times, then allowed to grow out for a number of years. This is what it takes to develop a juniper from a small plant.