yes, it is different (i used to do my own printing in the "wet" lab too, so i know exactly what you mean). there is nothing to recalculate, there is no reason why a 5x7 should be darker than an 8x10 (actually, if we talk about optical enlargers, it would be the other way around, wouldn't it?). it's just a malfunction/missetting in the printer used (or in the operator
)
maybe consider buying a printer and doing it yourself (that's what i did). it's more trouble, but it will bring back those memories (remember what it felt like to be in control?
). a decent photo a4 printer is quite cheap these days, and with ink and high quality paper it works out to a cost per print of about what i pay at a decent lab (or even less), not as cheap as some internet order ones, or some us prices i heard, but oh well... my printer is an epson r285 (the cheapest epson photo printer you can buy, i daresay, probably not much more than 50bucks in the us, with enough ink to get you interested bundled with it -- just enough, not more though
)