Much smaller will be ahrd to achieve with a hard drive inside unless using mini
hard drives (portable or even 1.8") and both would make it a more costly
device (especially the last options). 2.5" are actually dropping in price quite
rapidly nowadays but do not exist in volumes og 1TB and 1.5TB at this time
and people do need volume for which the option of 3.5" drives should remain possible.
Don't want the size... Pick the iSTAR Mini or Kaiboer NMT. They both support external hdd but are small due to not having internal HDD design.