Not understanding how you are finding MTB shoes bulky, most are very similar to road shoes, just with the addition of tread, MTB shoes which are bulky are normally those which are in the style of flat shoes.

If you are looking for something which looks like a running shoe, have a look at something like Shimano MT44's SH-MT44 - TOUR - CYCLING FOOTWEAR AND PEDALS - LIFESTYLE GEAR - SHIMANO