Why computers in the first generation were very large?

Digital computers do not need to be electronic, but have sometimes been implemented mechanically, hydraulically or pneumatically. To be fast, computers have to be electronic and the first devices used were valves (tubes). These are relatively large, and consume around 10 watts of power each, and since thousands were needed, computers were large and needed a lot of power. Transistors were next, once they became fast enough, and computers became much smaller. The next step was the integrated circuit ("chip") which comprises hundreds, thousands, or millions of interconnected transistors made in one (small) piece. These have continued to become smaller and more complex year by year.