Well Tagmatic units were primarily cavalry and a part only infantry.
Over the time the infantry regiments grew in numbers as the constant campaigning led to an increase of a imperial tagmatic infantry which accompanied the cavarly during the Byzantine reconquista.
Further to that, the increased number of Tagmatic troops led to gradual replacement of the part-time Thematic armies with the professional Tagmatic ones.
However the system was about to became a victim of its own success. The professional Tagmatic troops made the Empire secure of her enemies during the 11th cent.
As a period of relative peace followed the gradual replacement of the Thematic armies, the central authorities didn't see any need to maintain the great numbers of the Tagmatic troops and the Tagmatic troops were in their turn neglected and shrunk in numbers. This has been done partially due to the mistrust the non-campaigning emperors had for the army and its highly ambitious aristocratic officers.
The situation was put to test during the Seljuk invasion with the well known events.Just imagine the huge effort Alexius Komnenos has done in order to rally the scattered tagmata when he faced the Norman invasion,