All items get bigger the bigger the hall is.. while it makes sense for some items such as engines it does not make sense for weapons and sensors and colony pods and the like... the real problem however is that miniturazation actually makes items take MORE space (while also increasing the amount of space available on ships).
EX: when researching one level of miniturazation my cargo hulls gained 10 extra space, but my best engines changed size from 7 to 8.