Los antifit son los pantalones baggy, es decir, muy sueltos u holgados, de esos que hacen bolsas por todas partes.
Por otro lado, los pantalones loose fir son sueltos y cómodos.
En el mundo de la moda se conocen como baggy, yo lo dejaría así, en cursiva.

--------------------------------------------------
Note added at 9 minutos (2011-05-11 16:45:06 GMT)
--------------------------------------------------

Anti-Fit is one of the defining features of the Levi's® 501® Jeans, and as such has been around since the 501® jean was born in 1890. It's a way of cutting the rise of the jean in a straight line (as opposed to curved) which gives the 501® jean its recognisable 'top block' (the section of the jean that goes from the waist to the crotch). It came about as a result of trying to maximise every inch of the denim fabric in the late 1890s and early 1900s, and also as a result of taking the original Levi's® Overall and cutting them down to a pair of jeans. Overalls aren't cut to follow the body but rather to 'hang' on it, and so the 501® jean has that same cut - the 'Anti-Fit' cut that doesn't follow your body shape. There are many benefits to anti-fit: Greater comfort, less wear on the seat, and of course, the ability to look good when worn 2 sizes too big!