Unfortunately I've been unable to find the original study.
It seems that lately sodium bicarbonate has become the "wonder drug" of poultry. ugh
However, I do recall the mechanism of it. Sodium bicarbonate is known to cause ascites and gout in chickens. Especially in heavier meat breeds.
The increased salts are processed by the liver first.
While liver enlargement is mentioned merely as a part of ascites and gout caused by sodium bicarbonate, it is actually one of the first symptoms as the birds body works to balance out the ph and the liver filters the excess salts.
Thus, while sodium bicarbonate may not present fully as gout or ascites, it can cause enlarged liver prior to any clinical presentation of more advanced disease.
I've included two links which show the relation of sodium bicarbonate to gout and ascites and the "casual" mention of the enlarged liver as a part of the symptoms. In fact, since the liver is the first organ involved, it's one of the first symptoms to present.
In cases in which the level of sodium bicarbonate isn't high enough to ever develop into a more serious health condition, it can still affect the liver over time causing a chronically enlarged liver.
(I think I got all of that in the right order. It was something I was studying a few years ago in relation to duck management.)
http://www.poultrydvm.com/condition/gout
https://en.engormix.com/poultry-industry/articles/ascites-in-broiler-chickens-t35214.htm