Thanks for your query.
You have no reason to worry.
First of all - the x ray
report simply suggest that the size of heart looks smaller as compared to the size of chest. In persons who are tall & well build it is a normal finding.
Secondly there is no medical condition known to cause small heart & in most diseases of heart the size remains same or enlarges.
In my opinion you do not have "small heart".
In any case you are going to see cardiologist & there is no point in guesswork or believing in what somebody says in school.
Best of luck.