Magnesium malate would have a total weight that is high.

This is very confusing to most people, especially doctors.

A typical magnesium malate (one that I have) is 1000mg per tablet. Only 150mg of that is the magnesium portion and the rest is the malate portion.

All dry powders of mineral chelates/salts typically are reported as total weight. Some labels will detail the elemental value of the mineral--in this case magnesium--and others will not.

I explain elemental values here:
http://neurotalk.psychcentral.com/post20739-10.html
This post is from this thread:
http://neurotalk.psychcentral.com/thread1138.html
It has grown pretty long, but there are interesting questions on it, and it is a very valuable subject, so please try to read it.

I don't think you need to return your malate at this point. If it causes loosening stools for you, that would be a sign that not all is being absorbed. The SlowMag is magnesium chloride, and 2 a day of those are usually enough for most people. But one can go up to 4 a day if needed, and if you have normal kidney functions.