We are looking at ventricular hypertrophy here.
The human heart has a left and a right side. On both sides there are a ventricle and an atrium (1) (2). Basically the heart is just a bulk of muscle tissue responsible for circulation and blood flow.
As with any muscle contraction when there is a resistance against the working muscle this leads to muscle growth or hypertrophy. A bad example of such resistance is a high blood pressure. Due to the elevated blood pressure, the heart must pump and contract against a continuously present counter force and this leads to a heart muscle hypertrophy, or ventricular hypertrophy (3). When hypertrophy is due to, say, high blood pressure this hypertrophy or growth is pathological and eventually leads to muscle stiffness and heart failure.
In your case, there is probably also some heart muscle hypertrophy but it is non-pathologic and it can be considered as a healthy "heart condition". The principle is the same. When you workout your heart must work more efficiently compared to its resting state and pump more blood to the skeletal muscles due to increased demand for oxygen. Due to the advantageous heart muscle growth your heart is capable of circulating blood more efficiently.
Whether the heart muscle growth is pathologic or non-pathologic depends on the concept of afterload (4). In a healthy heart the afterload is minimal indicating that the heart "can rest" shortly after ejection or ventricular contraction. This results to beneficial hypertrophy. However, if a substantial afterload is present, which is the case in elevated blood pressure, the heart "can't rest" after contraction but instead is must maintain some level of muscle contraction which results in adverse muscle hypertrophy.
However, in extreme cases the workout can result in some health issues, called an athletic heart syndrome (5). This is still far less adverse than heart failure.