I can only assume you mean the 60ml tastes different than the 15ml. That would be caused by steep time. A 15ml is gone rather quickly, your 60ml would steep longer than the 15ml by the time you finish it, therefore adding steep time and changing the profile some.

If you prefer the mix more while it's fresher, I'd suggest mixing a max of 30mls at a time or perfecting the recipe to stand the test of time for larger quantities.

If it tastes a little different, consider this....

Mixing say 3.2% of a concentrate to make up 15ml you need to be incredibly accurate to ensure it is just that, 3.2%. There's a lot more leeway for those extras few rogue drops when you're mixing larger amounts.

The bottles could taint the taste (old reused/cheap plastic etc)

Steeping time. As I'm sure you're aware, flavours change over time as the juice steeps.

How old the concentrates are, they don't last forever. I always buy my concentrates from the same supplier when I can. Find a good one and stick with them.

As long as you have the same flavor percentages it’ll be the same. Except maybe a little bit different because using the same bottle for the same juice without cleaning it makes juice right off the shake a bit better. In my experience. Using a different bottle might make the most minute changes right off the shake.

You didn't mention exactly what happened - I assume the taste is different?

I can't really think why that would be - they should taste the same. Are you using a calculator to figure out the new numbers? Was the 60ml bottle clean? Could there be another factor like a gunky coil?

What exactly is happening with your mix? How can someone explain this to you if you don't tell us what the problem is?

What do you mean „what might be happening with my mix“? What you you think is happening?

Which flavors from which companies did you use? Some need to steep, but it doesn‘t really matter how much you mix of the same recipe as it will be the same. If you mix a gallon of Pepsi with a gallon of rum, it‘ll be the same as a quart of Pepsi with a quart of rum.

Okay now are you making the same amount but just put in in a different bottle? Or did you make more ML of the e-juice?
If you made more ML then, you must add more flavor, more nicotine, more VG/PG for more ML. If you add the same ML for a 15ml to a 60ml then it's likely not to turn out good. I do not think that you will taste the flavor at all much less the

I suggest you use an E-liquid calculator (Just search it on the web) to find out the precise ML you need to add. Each concentrated flavor has it's own percentage of mixing and that's how you know what to add. (It is not a "rule" but if you're just starting out I suggest follow these percentages and don't mix too many flavors together because it might turn out something which tastes like shit)

However if you have made experiments and found out this recipe on your own (by testing) then you need to calculate it yourself. If you add X amount for a 15ml and you want now 30ML then it's X*2 (you get the idea)

If it tastes different after scaling up the batch size, you've made either a math error or a measurement error. It's hard to measure flavors accurately for small batches.

You didn't actually ask a question 😂 I've had that happen tho, i mixed my 8ml tester of Spring, loved it, mixed up a 30ml and it tastes super strange. I can only assume i fucked up somewhere, either in the original amounts when i mixed my tester, or when i mixed the 30. Hard to say and now i have 30ml of a juice i don't love