It's not the size of the dog in the fight...
First of all I'm going to apologise in advance for my never-ending questions regarding "expressions". I am a person that uses a lot of colourful language and I'm finding it a great way to move along with my Spanish. I seem to be able to remember an 'expression' in Spanish much better than a plain old, you know, 'Yesterday I got up, I went to work, I was very busy.' or whatever.
I've looked up many 'Spanish expression' sites and some are extremely good but..don't always have what I'm after..
So just one more...perhaps...
"It's not the size of the dog in the fight.. it's the size of the fight in the dog.".
(Meaning don't be complacent or be fooled by small stature ..myself included! Grrrrrrr!)
And..thanks for all the input - I learnt many things 'yesterday'.
Gracias, son excelentes! Voy a anadir a mi repertorio!
(I'm just about to have a look at the link suggested for how to add accents)
Rather than being a standard English expression, this is a Mark Twain quotation. As such, it should be translated rather closely, rather than using an equivalent Spanish expression. I found this, which looks good to me:
[Lo que cuenta, lo más importante] No es el tamaño del perro en la lucha, sino el tamaño de la lucha en el perro (el carácter luchador del perro).
Not even remotely related to the implied context of the phrase that you offered but in some contexts "más vale maña que fuerza". Somewhat literally, know-how is better than (simple) strength.