It means you have a small heart, literally. The term "relatively" could mean anything because they haven't said in relation to what. Without the reference, it is difficult to understand what they are saying. If I say you have a relatively small heart, in my mind I could be thinking in comparison to an elephant. Maybe they are saying your heart is small for your body size. However, this is nothing to worry about because we all have different sized hearts. If it was a problem you would have noticed by now, so it's obviously big enough to do the job. An xray is hardly the best imaging technique for the heart anyway, you would need an echocardiogram to see the true dimensions.
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