Yeah I believe that is the case. I think Y is more important than X. If you did a

-resize 10000x

then it would do 10000 wide and scale the height to match aspect. So I think yours is equiv to

-resize x200

I believe too that if you use the mogrify command, which is exactly the same as convert but it works on the image supplied not save to a different one, then you can scale all the images in a directory with a wildcard so that they are all 200 high like this

mogrify -resize x200 *.jpg

at least your supposed to be able to. I tried that on some test images and its crashing because of the wildcard in it. Hmm not sure but its supposed to work. You should be able to convert all images from jpg to png in a similar manner too...

mogrify -format jpg *.png

Anyway, you get the idea, its pretty easy to do batch processing of lots of images at once this way. I know other tools can do this too but I don't think its so easy or so controllable as IM.