In each cell your are correct but the 48 v battery has 12 more cells so the total plate surface area would be greater- this assumes both batteries have the same same amp rating per positive plate & the same total kwh & we are taking about flat plate cells vs tubular design which typically has more surface area than flat plates but has some limits & some benefits.
Lets do this so I can better understand what you have in your lifts. Go out to your fleet of sit downs & get the battery model number from a unit with 36 v battery & one with 48v & measure the battery trays length, width, height (measure height to top of lifting eye) - the model number maybe on a battery tag with a bar code or stamped in the case. The model will be in this format - for 36 volt battery 18-XX(x)-yy; 28 volt - 24-xx(x)-yy. If you cannot find it on the case - lift the top cover, disconnect the battery, remove all rings, watches, were eye protection & don't touch any of that white stuff you may see - that has no street value but it will eat the heck out of anything with cotton - it is an acid sulfate. Find the red cable, go to where it is connect to the battery, remove the (or more than one) plastic inter cell connector cap(s) closest to the pos. cable. The model number should be stamped on top of one of the inner cell connectors. If you can't find it then teh connectors may have been replaced & teh tech - did not restamp the connector (supposed to - but it is 2011).
Another idea get a hold of your forklift dealer rep & have him bring in the rep from the battery company that made the batteries & request he bring in literature & other information on batteries for your better understanding - you'll get enough information that will challenge the number of pages in A.H.'s book "Mein Kampf" (at one time it was rated the worlds largest book). It has been my observation, that few forklift guys understand batteries & the battery guy is like a "Maytag" rep. they luv an audience & Q & A sessions - they can get way too technical at times but tell him you don't want to know how to make one just a better understanding of what they are & what options are available to make your job easier. And if they don't have let's say a tubular battery or other options - like MF, etc. they will tell you nuttin' but bad about them Heck you might get a lunch out of the deal.
Just "Free Food & Info For Thought"

PS - I was a lift truck sales/marketing guy for several decades (like 4+1 year) & I wanted to know about batteries & options so I could better consult with my prospects & customers - I am a tech oriented guy - degree in engineering (never engineered one thing for a company I worked for only to get a grade in college or some "back yard engineering).