You have to know the differences between totalsize and sumsize. If you have a big saveset that spans in two or more volumes, you will have several savesets with different size. You can check this adding "sumflags" in your mminfo query.
Sumflags could be t, m, or h, means tail, medium, and header.
Now, if you ask for sumsize, you will obtain the list of parts with its own sizes:
saveset (header) -> size of header
saveset (medium) -> size of medium
saveset (tail) -> size of tail
But if you query about totalsize, each saveset shows the size of the whole saveset:
saveset (header) -> size of saveset
saveset (medium) -> size of saveset
saveset (tail) -> size of saveset
It occurs if you add in your query some parameter thas tells mminfo show every part of the saveset, for example, sscreate (each part of the saveset have its own date of creation). If your query not have this kind of parameters, then totalsize is the same than sumsize.
On the other hand, you can use sumsize(20) and mminfo shows size in bytes... now you can be sure that all values are in the same unit.