Were you explicitly clear in your requests for quotes? As in how many tiers, shape, decoration, what kind of frosting? Did you ask them to define what they meant by serving size? They may very well both be telling the truth, but may be operating under different assumptions unless you you were VERY detailed in what you asking for and clarifying certain points.
For example, a standard wedding cake serving is 1″ x 2″. Most bakeries are going to quote based on that serving size. A grocery store? Probably not. Also, my family loves cake – there is no way in hell 1″ x 2″ is going to be a “serving” to my family. But based on that serving, a 14″ square tier will serve about 98, 10″ tier 50 people, and 8″ tier 32 people (round tiers just slightly less) So the bakery would be accurate in their assessment assuming everyone involved is talking the same size serving. And if they were talking about a coffee serve size, that is 1″ x 1″ and a 14″ tier serves around 200.
Additionally, fillings cost more – did one assume just a plain cake and one a filling?
Fondant costs more – did one assume buttercream and one fondant?
Intricate decorations cost more – did one assume a more intricately decorated cake than the other?
Were delivery costs included in this quote?
And just generally speaking, grocery stores are ridiculously cheap and likely will be much cheaper than a local mom & pop bakery. The grocery store has a lower overhead comparatively speaking to their revenues because their fixed costs are spread out amongst their national chain, they are able to get away hiring less skilled workers, they can outsource baking to a large facility, they deal in mass quantities which also spreads out fixed costs and allows greater turnover of product because the level of customization isn’t there (you generally will have a much greater flavor and decoration variety at a bakery vs. grocery store). And while some grocery stores have great bakeries, I find they tend to use inferior ingredients and you can taste it in their baked goods – quality costs money. I’m not familiar with this particular chain of grocery stores, so I’ll withhold commentary about the quality for them specifically.
So other than Publix estimating VERY generous serving sizes (three large tiers like that is going to be way too much for 50-75 people unless you’re doing only cake and punch and allowing for extra large servings or seconds, saving the entire 8″ tier for yourself, or sending cake home with people – in what is viewed as a more traditional 2″ x 2″ serving size a 14″ square tier will serve around 50), both are probably telling the truth.