so yeah, i made this HUGE scenery object, but the mapmaker only showed like one fifth of it, could this be caused by lack of free ram, etc.?

Well I have made a test map with a scenery item 2048x1536 and everything worked completely fine, in both the map maker and playing in it. So unless your image is considerably larger than this, it is most likely a problem with your PC, as you say, possibly a lack of RAM, or maybe something to do with the gfx card. I can't really help you there unfortunatly.