The availability of fitted clothing was quite rare, and very expensive, in the 19th century. "Readymades" and most certainly military issue clothing, was indeed quite baggy. Edison film of the Spanish American war shows enlisted men in very loose fitting uniforms. Officers would tend to have better fitting outfits, as they were expected to provide their own. The quartermaster shop trousers are very correct, made to MILSPEC requirements.
Posed/studio photos of the era tended to use fitted clothing retained by the photographer, and is not really indicitive of actual field wear.
Stripes on trousers were indeed a sign of rank. Per the 1872 regulations, privates would have none, corporals a 1/2" stripe, sergeants 1", field grade officers 1.5", and those of general rank 2"