I think there is a lack of information on breeding tarantulas in the wild!

If you think about the male tarantula on maturing will make he’s sperm web and load up and then off he goes to find a female
Which is he programming so if he finds a burrow he will go in and mate with the tarantula inside the burrow.
So if you think about it the mature male does not know the difference between female and male or the size of the other spider he’s just there for one thing and one thing only to insert.

I have done that where I have mated a mature male and immature male
They both did the tapping thing the male did he’s hold the boy down and inserts.
I mated them about three times. :blink:

But it does not mean you can take 5cm female and put it with 15cm male
That will be silly and CAN LEAD TO TEARS.
So it will be good to take Scarabs advice determining factor would be the sclerotisation of the spermathecae

But then to do your own thinking and if you feel happy and sure of your self then there is nothing stopping you from attempting this!

If you look at Baboon spiders they have egg cases when they are still juvies
And if you think about this if a female takes 4 years to mature before she can get breed buy that time all the males are dead so I really think they can be mated a little bit smaller than normal.

Also if you have a agro species surely mating the juvie female with a larger male will keep her from eating the male and also allow the male to get in there and get a good couple of inserts .

i will try it and post on this topic what the result was.