I discussed this earlier. While intelligibility between Spanish and Portuguese is high, it is not equal. Portuguese speakers often say they understand Spanish better than Spanish speakers say they understand Portuguese. The only intelligibility study found that Spanish speakers had 50% intelligibility of Portuguese but Portuguese speakers had 58% intelligibility of Spanish.
The link shows you why this is so. There are two factors. One is that Portuguese has a more complicated vowel system with nasalized vowels. The other is that Spanish is more staccato, with words being separated more in speech while Portuguese is more connected with words flowing together. Spanish speakers are lost in the vowels and can’t pick the words apart from the river of speech.