No, memspace is just a description of the buffer in memory (i.e. where read elements will go). If there is no data conversion, then we read directly into the user supplied buffer. If there is data conversion, we use a 1MB buffer to do the conversions, but we still use the user's buffer for reading data in the first place.
Also, you can adjust the 1MB default conversion buffer size. (see H5Pset_buffer)