That's not how it should be! The tightest position for the chain is when front sprocket centre, swing arm pivot and rear axle are in line (so it should get tighter when you sit on it!) - in fact the absolute best way to tighten your chain is to arrange the three points are in line and then have a small bit of play at chain run tightest point but it's a bit of hassle so usually a figure is given for when bike is on the stand. So you kind of gently pull down on chain in middle of bottom run then measure from that point and push chain up which should give a figure of around 5cm. Hope that helps.