As mentioned, the Army clothing was made in a few standard sizes. But in the 1830s and 40s the mania was for "fit" as in the civilian fashion, viz. NO folds, etc. So it was retailored to fit the man.
When possible, the fatigue dress of recruits, etc. was fitted before they were dispatched to their units. From 1839:
Augustus Meyers joined the Army in 1850, while the sky-blue fatigue jackets were still employed, and noted his first duty at recruit depot was to carry his fatigues to the post tailors to be fitted before he could fall into the ranks on parade...
"As the soldiers' clothing was made up in men's sizes only, there were none to fit the boys. I believe there were about six different sizes in shoes and three or four in clothing. The smallest size in clothing, No. 1, was issued to me, and I was sent to the post tailor. He took my measure and altered the great coat, jackets and trousers. He also put some white braid on the collar and sleeves of one of my jackets. The cost of these alterations were deducted from my first pay due. It was moderate enough, for the tailor's price as well as those of the laundress and the sutler were fixed by the Post Council of Administration. With the shirts and drawers I was obliged to get along without alterations, voluminous though they were. The shoes were too large for me also, but the thick woolen socks helped to fill them. No dress coats were furnished to the boys while they were on the Island. We only got those after joining a regiment.
In about a week my clothes were ready. I arrayed myself in my new sky-blue uniform, experiencing a boy's pleasure in a new suit and some pride in what I considered my fine soldierly appearance. We were not allowed to keep any citizen's clothing, so I sold my clothes to a Hebrew "Old Clo' Man" who often visited the island for that purpose. He paid me a dollar for them, the possession of which made me quite popular with a few of the boys who showed me where we could buy pies and ginger-pop at the sutler's store."
An 1851 United Service Journal correspondent noted that by that time the recruits were charged as follows for basic tailoring...
Fitting and lining a fatigue jacket, $2.00
Fitting two pair wool overalls ($1.25 ea.) $2.50.
Fitting a uniform coat, $2.00.
Fitting three pair cotton overalls (.50 ea.) $1.00
Altering fatigue cap, .25.
Total $8.25.
The Regular Soldiers were not provided with the Uniform (cap, coat, etc.) until they joined their Regiments in active service. As mentioned, the coats too were tailored to fit.
For example, During their years in the seat of war in Florida, the 4th Regiment of Artillery did not bother about the uniform at all, wearing only fatigue dress, etc. On leaving that Territory and appearing at the Trenton encampment in the summer of 1839, the regiment was provided with uniform coats for the upcoming 4th of July... The men were described in camp furiously stitching away to get their coats properly fitted for the big day...
An officer observed in 1840 the enormous trouble of the receipt of the companies annual clothing, "everything is overwhelmed and prostrated for about a month, until said clothing can be inspected, pulled to pieces, cut over again, and made up anew, to suit the forms of its respective wearers."
One of the negatives of the fitted garments was that they wore out faster, being taken on and off, etc. I've seen notice the the officers who were obliged to wear their uniform coats daily, on parades, could expect to wear them out in a few years (They otherwise wearing their frock coats, etc. when not on parade, generally). Enlisted men had to wear them on parade, and also for guard and other duties other than fatigue, etc.
That all being the case, it appears many of the troops in active service, as in Florida for long periods, simply employed the fatigues as provided, without concern for their appearance. A common description from 1839 of some 4th Infantry men moving about from Florida and the Cherokee country, etc.
From 1840s photographs, a Mexican War period recruit, in an untailored jacket...
A pre-1847 sergeant in tailored fatigues...
It is also evident that many of the tailored jackets had the clumsy shoulder-straps removed. Like this lad... (notice that he's placed what appears to be hooks and eyes along the edge of the jacket front, evidently to take the strain off the buttons of his tight fitted garment...
From Sarasota, Fla., 1840, an infantryman with the shoulder straps removed (leaving the button though).
Besides fitting the jackets, it was mentioned by some veterans that some units had the men's trousers fitted to include "gaiter" bottoms with buttons near the ankles in the fashion of the 1830s-40s.