Were people back in the day just smaller and not as muscular? Was it food related, did they not have enough food for their soldiers?

Yes. The average height of men in the 1940s when compared to men today was not as markedly different than the weight, however.

Malnutrition among children had been a serious problem in some regions of the United States during the Great Depression, and generally, obesity to the extent of being disqualifying for military service was uncommon enough to the point that medical examiners following the provisions of Mobilization Regulation (MR) 1-9 (the regulation adopted on 31 August 1940 that replaced MR 1-5, 5 December 1932 with change 1, 29 July 1938, and that carried through the wartime period) were permitted to use their own discretion to an extent when they encountered cases while examining the weight of registrants.

The regulation covering medical standards for accession into the Regular Army, National Guard, and Organized Reserves for enlisted men during the peacetime period between the world wars, Army Regulation (AR) 40-105 (adopted on 21 May 1923 and superseding AR 40-105 of 20 June 1921, and revised up to change 5, 17 August 1940) was stricter than the "mobilization" standard.

Section V of AR 40-105 covered measuring the height, weight, and chest size of applicants for enlistment. At standard measurements, applicants had to be between 64 and 78 inches (5’4” and 6’6”) in height and weigh between 128 and 211 pounds, depending upon the height; minimum measurements were 120 to 175 pounds, depending upon the height. "Mobility" was the difference in chest circumference when inhaled versus exhaled.

A = Standard; B = Minimum

A.) Height	A.) Weight	A.) Chest at expiration	A.) Mobility	B.) Height	B.) Weight	B.) Chest at expiration	B.) Mobility
64 (5'4")	128	32	2	64	120	30	2
65	130	32-1/4	2	65	121	30	2
66	132	32-1/2	2	66	122	30	2
67	136	33	2	67	124	30-1/2	2
68	141	33-1/4	2-1/2	68	126	30-3/4	2
69	148	33-1/2	2-1/2	69	128	31	2
70	155	34	2-1/2	70	130	31-1/4	2
71	162	34-1/4	2-1/2	71	133	31-3/4	2
72 (6'0")	169	34-3/4	2-1/2	72	138	32-1/4	2-1/2
73	176	35-1/4	2-1/2	73	143	32-3/4	2-1/2
74	183	36	2-1/2	74	148	33-1/2	2-1/2
75	190	36-3/4	3-1/4	75	155	34-1/4	2-3/4
76	197	37-1/4	3-1/2	76	161	34-3/4	2-3/4
77	204	37-3/4	3-3/4	77	168	35-1/4	3
78 (6'6")	211	38-1/4	4	78	175	35-3/4	3

Filipinos (i.e., Philippine Scouts) were given different minimum standards in certain categories:

Height	Weight	Chest at expiration
59	100	28-1/2
60 (5'0")	101	28-3/4
61	102	29
62	103	29-1/4
63	105	29-1/2
64	107	29-3/4
65	110	30
66	113	30-1/4
67	118	30-1/2
68	124	30-3/4
69	127	31
70	130	31-1/4

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Section III of MR 1-9 covered measuring the height and weight of registrants.

14b. Examining physicians should use discretion and judgment in accepting registrants with slight variations in the ratio of height, weight, and chest measurements indicated in the table. Minimum and maximum height are absolute, but when the weight is disproportionate and is believed to be due to some temporary condition, proper allowance may be made provided if it is the opinion of the examining physician that the variation is correctable with proper food and physical training. But no registrant may be accepted whose weight is less than 105 pounds.

Men who were taller than 78 inches, less than 105 pounds, or "overweight which is greatly out of proportion to the height if it interferes with normal physical activity or with proper training," were to be rejected.

A = Standard; B = Minimum

A.) Height	A.) Weight	A.) Chest at expiration	B.) Weight	B.) Chest at expiration
60 (5'0")	116	31-1/4	105	28-3/4
61	119	31-1/4	107	29
62	122	31-1/2	109	29-1/4
63	125	31-3/4	111	29-1/2
64	128	32	113	29-3/4
65	132	32-1/4	115	30
66	136	32-1/2	117	30-1/4
67	140	32-3/4	121	30-1/2
68	144	33-1/4	125	30-3/4
69	148	33-1/2	129	31
70	152	33-3/4	133	31-1/4
71	156	34	137	31-1/2
72 (6'0")	160	34-1/4	141	31-3/4
73	164	34-1/2	145	32
74	168	34-3/4	149	32-1/4
75	172	35	153	32-1/2
76	176	35-1/4	157	32-3/4
77	180	35-1/2	161	33
78 (6'6")	184	35-3/4	165	33-1/4

The next edition of MR 1-9 was issued on 19 April 1944, it superseded the previous edition, 15 October 1942, which included changes 1, 22 January 1943, and 2, 23 February 1943, and section I, Circular No. 137, War Department, 1943. The standards of height and weight remained the same throughout all editions, and paragraph 14b of section III was unchanged. Various other standards were used during the wartime period, including AR 40-100, "Miscellaneous Standards of Physical Examination," dated 16 November 1942, and AR 40-110, "Standards of Physical Examination for Flying," dated 8 December 1942.