It's worth remembering that pet shop employee's however intuitive they are on pet care and housing sizes, have to follow rules same as everybody else in the workplace. It is a business and if they don't bring in money, the employee's have no job. If they don't follow rules set out by the managers, store owners etc, they can quite easily receive warnings and eventually the sack. Working in the animal sales industry is not easy and not for everyone, it's an extremely difficult balance. Working for a franchise is even more difficult.
I don't work in a pet shop but do work in an aquatics, and I know what our weekly target is - that is the minimum we have to take otherwise we start taking losses which of course puts us at risk of closing and no jobs. This often means we're pushed and under pressure to sell certain stock, whether it's to clear it or for other reasons.
That being said, if you have a particular cage in mind, be direct about it and that you're not willing to budge. The problem is, working in the animal sales industry 95% of customers do not have the compassion nor the slightest desire to get the biggest/best housing for their pets. They want something compact, aesthetically pleasing and often the cheapest thing. Businesses just can't get by on the 5% who go to extra lengths for their pets. No demand = no supply.
You may find pet shop employees going home with dismay every night because nobody wants to buy spacious, suitable accommodation for their pets, so the store doesn't tend to stock it for those few that do because quite simply it doesn't sell. You'd be surprised how frustrating it can be. All businesses can order stock in, so talk to them, ask what they have available that's not in store. If you have the receipt, absolutely go back and talk to them about swapping for something you feel is more suitable. You can still try now. They do have more range of cages, available to order in. Hope this helps.