As a couple of commenters noted, "tight" does not have a clearly defined, widely recognized meaning in this context.
That said, as a software developer, we use the word "tight" to refer to a software product that accomplishes the job with the least amount of code. That is "efficiently designed" in a sense comparable to saying that a mechanical device accomplishes its purpose with a minimum number of parts.
So for a website, if, for example, it had one screen to both enter new customers and to update information about existing customers, instead of having a "new customer" screen and an "update customer" screen, that would contribute to calling it "tight". Or if the same graphics and banners were used consistently throughout the site. Etc. If the whole design of the website was like this, if it did the job with 10 screens when others would use 30 and with 20 icons where others would use 40, we'd call it a "tight website".