when does RSA_eay_private_decrypt called the rsa_mod_exp function?

I summary the function call in rsa-encryption of openssl. We can found that

if (p!=NULL && q != NULL && dmp1 !=NULL && dmq1 != NULL && iqmp !=NULL), 

RSA_eay_private_decrypt() and RSA_eay_private_encrypt() will call rsa_mod_exp(Is this the Chinese Reminder Theory(CRT) version?).

My questions are:

1. In publick encrypt or public decrypt, "e too small" is the reason that only call bn_mod_exp function?
2. In private_decrypt or private_encrypt, as I know, d is as big as modulur n. when does rsa_mod_exp() is called? and when bn_mod_exp() is called?

via Chebli Mohamed