You are comparing consonants at the beginning of a syllable with consonants at the end. Consonants at the beginning are fortis, while those at the end are lenis. This is a very general difference in English and in some other languages. Saussure called syllable onset, fortis, consonants explosive, and he called syllable offset, lenis consonants, implosive (in Course in General Linguistics).
Fortis consonants use more muscular effort, and consequently for the dental fricatives, the tongue is pressed more forcefully against the upper teeth. This makes the obstruction to the stream of air through the mouth smaller and makes it last longer. More air pressure will build up in the mouth, behind the obstruction, and when the consonant is released, there will be a larger explosion of air than for the corresponding lenis consonant in the offset of a syllable.
Lenis consonsonants are more subject to lenitive changes, such as assimilation, than fortis consonants. In English "tenth", the final "th" can assimilate to the preceding (nasal) stop to become a dental stop.