Mouthpieces are usually harder to blow because they have a larger tip opening than you can comfortably manage with the reeds that you are using. On alto I mostly play pieces between 0.076" and 0.080" and I use 2 1/2 reeds. If I used 3 reeds I would struggle a little to sustain a good sound. Similarly if my mouthpiece had a bigger tip opening - more than 0.090" - then I would find it tough playing a 2 1/2 reed.
On the other hand I have a Jumbo Java A55 mouthpiece which is 0.097" but it playable on my 2 1/2 reeds as it has quite a large baffle.
May be an idea to try lighter reeds on the mouthpiece that you like. On the other hand not all mouthpieces are well made so resistance can be due to workmanship/design issues.
