Most base layers are slimming in some regard, as they need to remain close to the skin to prevent warmth from escaping.
Nike, on the other hand, refers to its base layers as a ‘secure fit.’ This is because they offer enough snugness to uphold their thermal capabilities without ever feeling too tight.
The sleeves, for instance, sit looser against the skin than some other base layers, for greater freedom of movement. It’s because of this you could likely get away with wearing a Nike base layer as your top layer, if you like. Weather and temperature permitting, of course.