It's in the verse you quoted, but it isn't apparent. You'll have to look at what was actually meant by the words used. Some versions/tranlsations make it obvious, but the NIV quoted below does not. I don't know why they chose a different word for narrow in verse 13 as opposed to verse 14, but narrow in verse 13 literally means narrow. Narrow in verse 14 means difficult as if by affliction or persecution:
ESV
13“Enter by the narrow gate. For the gate is wide and the way is easy
a that leads to destruction, and those who enter by it are many.
14For the gate is narrow and
the way is hard that leads to life, and those who find it are few.
NIV
Matthew 7
13“Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it.
14But small is the gate and
narrow the road that leads to life, and only a few find it.
G2346. thlibó
Strong's Concordance
thlibó: to press, afflict
Original Word: θλίβω
Part of Speech: Verb
Transliteration: thlibó
Phonetic Spelling: (thlee'-bo)
Definition: to press, afflict
Usage: (a) I make narrow (strictly: by pressure); I press upon, (b) I persecute, press hard.