SendSX is extensively tested, and works fine for any size of MIDI message (tested e.g. with 6 megabyte sized messages).
However, some MIDI drivers are known to be flawed in that they cannot handle "large" MIDI messages. So my first guess is that this is the cause of your problem. Please give it a try with SendSX -- to be 100% sure, you can throttle the data stream by lowering the speed in the options.
My best guess, however, is that your MIDI interface and/or its drivers are the problem. Maybe updating the MIDI drivers already solves the problem. You can browse some other topics in this forum for other people who solved their issues by buying a new MIDI interface.
With respect to Java, I assume that it'll behave more or less the same as SendSX, as, coincidentally, both MIDI implementations are programmed by the same guy (me)
Let us know how it works.