When you have small (thin) object selected.. you have bounding box with only corner anchors, and its impossible to change its size (horizontal or vertical) with hand.. look at attached image "smaller object selected" there are illustrator and indesign side by side..
in Indesign its works perfectly - even on small object its possible to change height without changing width..
In illustrator you need to zoom in many times to see those middle anchor boxes.
PLEASE make all those 9 bounding box anchors visible all the time, not depending from object size.