I can't believe nobody mentioned the Roland BK-5. I have an E-50 myself (two generations prior) which is slightly thicker and heavier due to the touchscreen.
The BK-5 is not as fully featured as a PA600 or PSR-s750. But Roland's arrangers pack some great sounds and styles in the smallest, lightest package of any MOTL arranger. I think it weighed about 17 lbs and a compact form factor too.