In the beginning of generic software - software that could be used by anyone - a program easily fitted on one or more floppies, even the ones of 'just' 160Kb. A virus via that medium spread either as a boot sector virus or attached to executables. Internet did not exist or was not widespread in these years - we are talking about the late 70's early 80's. But Bulletin Board Systems were abundant. Viruses spread attached to programs (Trojans) via these BBS's. They were extremely small, at least compared to the contemporary viruses, and the payload was single tasked: either overwrite a Boot sector or attach to a file. No sophisticated mechanisms were present in these viruses.
Programs and their supporting files soon no longer fitted on a few floppies and as a result viruses that made use of spreading via floppies disappeared more or less. As soon as you needed more than one CD to install your program Boot sector viruses were no longer an issue.
In the same time the Compact Disk (CD) became a very popular storage medium, making it virtually impossible to contaminate programs on that disk. At least as long as the software manufacturer paid attention. Also the Internet grew rapidly and became a public utility of mondial proportions: mid 1990's. As a result of these two developments virus coders needed a new vehicle to put their stuff in the wild. This made e-mail one of the most popular means to spread a virus. Of course viruses are still spread via floppies and files sent through mail or shared amongst friends etc etc. But the hyped Internet was a prime target of many script kiddy's building their thing in their bedroom. As a result a virus to have an effective payload became larger and larger and around 2002 sized up to 600Kb. Compared to 32K in the early years. Components as mail engine, polymorphic, binary devices are very common but result in relatively bloated viruses. Until now virus are not very intelligent. Most of them make use of security leaks of one particular (operating) system.
As by their nature to elude detection they should stay relatively small and thus cannot contain AI to adapt to a changed environment or sophisticated detection scheme. Unless you design a virus that get its information and resources from other sources: tapping from repositories on the basis of need.
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