The "small tsu" ッ is not pronounced tsu, but rather represents gemination.
The rule of thumb is that a final D gets transliterated as ッド (and a final T as ット)
- bed → ベッド
- pad → パッド
- good → グッド
- god → ゴッド
As you probably learned, this is part of a set of rules of thumb, which deal with final consonants in transliteration, since (except for ン) there are no kana without a vowel at the end.
As Japanese phonetics allow for /u/ to be almost silent, naturally kana from the /-u/-column in the kana chart (e.g. ク, ス, フ, etc.) would be good candidates to simulate final consonants. However, for /du/ or /tu/ this does not work, as these are pronounced zu and tsu and so here one uses the kana from the /-o/-column instead.
I guess the small tsu ッ is added to make the ド sound more "dry", i.e. more like a consonant and less like a syllable.