Presuming both of the refrigerators are 12 volt compressor type fridges (not propane powered) I agree that a 7 cu. ft. refrigerator speccing the same draw as a much smaller one seems suspicious. If it WERE true, (and presuming the 7 cu. fit one is not the size of a barn due to miles of insulation), then the 7 cu. ft. one would have to have a much higher percentage duty cycle, meaning it would use more energy in the end. Here are a couple of examples of what I mean:
Example 1, from my Vitrifrigo catalog:
Vitrifrigo, a common brand of good-quality 12 volt compressor refrigerators, does have much higher draw spec on the 8 cu. ft than the 4 cu. ft. (they don't have a 7). This tells me they are using an appropriately larger compressor (probably a Danfoss 50 as opposed to a Danfoss 35). I would expect these refrigerators to have similar duty cycles (maybe a bit more for the larger one). By duty cycle I mean what percentage of a 24-hour period it's actually running, vs. coasting.
4.2 cu. foot = 3.8 amps. At 50% duty cycle this would be 45.6 amps used in a 24-hour period.
8.0 cu. foot = 5.38 amps. At 60% duty cycle this would be 77.5 amps in a 24-hour period.
5.3 cu. foot = 5.38 amps. The 5.38 figure tells me it's the same compressor as the 8, and so it should have a smaller duty cycle, say 40%. That would be 54 amps per 24-hour period.
A company that theoretically uses the same compressor for a 4 cu. ft. fridge and a 7 cu ft fridge (hence same amp draw listed for both).
Theoretical 4 uses 3.8 amps. At 50% duty cycle this is 45.6 amps per 24-hour period.
Theoretical 7 uses 3.8 amps. At 75% duty cycle this is 78 amp hours per day.
So as you can see there sort of isn't a "free lunch" on refrigerator draw. If a larger refrigerator is specifying a lower draw, that likely just means that they are using a smaller compressor, BUT it will have to run more to remove heat from a larger "box." The only way around that would be to have it be disproportionately large on the outside (more insulation), but then it would likely not fit reasonably in a van. There is also the chance of more wear on the compressor if a smaller on is used in a larger box (they have figures for compressor/box sizing -- or at least Danfoss does -- so this can be determined).
I think most refrigerator makers do not specify duty cycle because who knows what your ambient temperature will be. However I WISH they would specify that for a set temp, say 77º. Having that figure along with the hourly amp draw of the compressor would allow one to plan for a power system, plus compare insulation value of various refrigerators.
PS: In general, when planning for your power system, think about three things:
1) What sort of draw and for how long in a 24-hour period on each appliance.
2) How much power storage capability do you have (batteries) (don't plan to draw them down 100% or they will die very prematurely).
3) How are you planning to "put back" power you have used, and how often.
With these three pieces you can get a basic perspective on your use. Two opposite examples:
A) Parsimonious boondocker.
1) Uses 85 amp hours in a 24-hour period
(refrigerator = 50; computer, etc. = 15; lights and water pump = 10; misc. = 10)
2) Has a 300 amp hour battery bank, so the daily draw uses around 28% of that. Hence 2-3 days without charging (or with meager charging due to clouds etc.) are not an issue.
3) Has 200-300 watts of solar to "put back" the power used. Can be supplemented by occasional driving (alternator will reliably charge batteries back up to 85%).
B) Luxuriies of Home folks
1) Uses 300 amp hours in a 24-hour period (luxuries of home
2) Has a 300 amp hour battery bank, so the daily draw would be 100% of that (not acceptable, but see #3)
3) But is plugged in every night (110 charger), and/or runs generator to charge (110 charger).
Both of the above scenarios "work," but the Parsimonious Camper has to watch usage carefully (or change system). The Luxuries of Home folks have to plug in every night and/or run generator. So the main thing is to match the system to the style (and of course there is middle ground).