The whos result is misleading for this comparison. For the following discussion the numbers cited assume a 64-bit system and that the overhead for a MATLAB variable is 124 bytes (this is approx).
First, every MATLAB variable has an overhead structure called an mxArray that contains information such as class, size, data pointers, sharing pointers, etc. So for variable "a" above, the data for "a" is only showing 1 byte, but the mxArray for this variable is taking up an additional 124 bytes of memory. So the total amount of memory actually used by variable "a" is 125 bytes. (actually probably more due to memory alignment effects)
For the struct testStruct, it too has this mxArray overhead. So there is 124 bytes right there. Then the data area of this struct is actually mxArray pointers, another 8 bytes. Then there is memory for the field name "a", so another 2 bytes minimum for that. Then the data itself is actually a MATLAB variable, so the overhead and data for that come to another 125 bytes. So the total for the struct testStruct is actually 124 + 8 + 2 + 125 = 259 bytes.
Bottom line is that you can't really compare the whos result of structs to numeric variables since whos does not tell you the whole memory story, it only reports on the data areas.
