The circumstantial evidence for this POV is compelling, as homers declined by 21% (1,358 to 1,067) in 1933, then jumped 26% (1,067 to 1,344) in 1934. (So if we adjust Foxx's 48 homers up by 26% in 1934 conditions, we have 60...)
Looking at sluggers of this era, especially if one focuses on 1933, it is easy to become convinced that there were a few modern-era type sluggers mixed in with a lot of singles and doubles hitters.
But before this conclusion is made, one needs to take into account what the ballparks were like in 1933. Most fans are aware of the numerous short-porch anomalies of the era:
* 290' RF foul line in Cleveland's League Park (but with a 45' fence)
* 296' RF foul line in Yankee Stadium
* 281' RF foul line in Philadelphia's Baker Bowl (but with a 55' fence)
* 296 RF foul line in Brooklyn (with a 19' fence)
* 300' RF foul line in Pittsburgh's Forbes Field (28' foot fence)
* 258' RF line in the Giants' Polo Grounds (also 279' down LF line)
* 310' RF foul line in Sportsmans' Park, home of the Cardinals and Browns (33' fence)
* 328' RF foul line in Washington's Griffith Stadium (31' fence)
* 320' LF foul line with a 31' fence in Fenway Park
You can see why left-handed batters had such a huge advantage in this era, and why right-handed Jimmie Foxx's 48HR/163RBI performance of 1933 has to be considered one of the greatest slugging displays, if not THE greatest, of all time.
Few fans, though, are aware of the total picture, which was decidedly against sluggers, and would probably cause current day players to go on strike until the parks were made smaller. Going in alphabetical order:
Boston's Braves Field: 359' down LF line, 417' to CF, 364' to RF -- much bigger than any current park
Boston's Fenway Park: Superficially similar in that there was a 320' LF foul line with a 31' fence, but CF was an unreachable 468', with the distance as much as 490' in deep RCF. RF foul line at 325' (and dropping away quickly) about 25' farther than the current "Pesky Pole". This is not the Fenway of Big Papi.
Brooklyn's Ebbets Field: This was larger in some areas than the famous Ebbets Field of the 1950's: 353' down the LF line, 415' to the deepest reaches just right of CF, 378' to RCF, and then the 296' short porch (19' wall) down the RF line
Chicago's Comiskey Park: A cavernous park with 362' foul lines and a 450' distance to CF
Chicago's Wrigley Field: 364' down LF line, 436' to CF, 383' RCF -- but only 321' down RF line, greatly favoring lefties like most other parks
Cincinnati's Crosley Field: 339' down the LF line, 400' to CF, a discouraging '377 down the RF line
Cleveland's League Park: A prodigious 374' down the LF line, and distances as far as 467' in deep LCF, 420' to dead CF. Only 340' to RCF and 290' down the RF line -- but don't forget the towering 45' wall there!
Detroit's Tiger Stadium: Forget the version you may know about. This one was a reachable 339' to LF but 464' to dead CF and a too-healthy 367' down the RF line
New York's Polo Grounds: Yes, 279' down the LF line and 258' down the RF line -- but close to 450' in LCF and RCF
New York's Yankee Stadium: 301' down the LF line, but quickly falling away to 402' in dead LF, 461' in LCF, and 490' in deep LCF. 470' in CF, and distances 417'-429' in RCF before diving in to the 296' porch down the RF line
Philadelphia's Baker Bowl: Here's one stadium that todays' players' union might accept, although the 55' foot wall + screen down the 281' RF line might be an issue. The park was a healthy 342' down the LF line, and 412' to CF.
Philadelphia's Shibe Park. This was Foxx's home and another park where modern-day slugging totals might be possible, unless you hit the ball to CF alot: 330'-335' down the lines, about 385' in the gaps, but 468' to CF.
Pittsburgh's Forbes Field: A hefty 365' down the LF line, quickly dropping off to above 400' moving towards CF, with distances reaching as high as 457' in deep LCF. Just 300' right down the RF line -- but with a 33' screen, and the fence angling quickly back to a 375' distance in dead RF, then to over 400' in deep RCF.
St. Louis' Sportsman's Park: Another more modern park in terms of HR reachability: a very healthy 351' down the LF line, 445' to CF, but just 310' down the RF line (with a 33' fence)
Washington's Griffith Stadium: The 407' distance down the LF line would probably have most right-handed batters picketing the stadium nowadays, although there was an area in LCF where the distance was "only" 383'. About 420' to CF, but a friendlier 328' down the RF line, probably unnecessarily defended by a 31' wall. Also, the RF wall angled away from the playing field, so you better hit it high and right down the line!