Krav maga and aikido are two very different modern martial arts that have become popular options for self-defense training. Krav maga was developed by Imi Lichtenfeld in the 1930s and '40s, and adopted by the Israel Defense Force as its unarmed combat system. Aikido is a Japanese martial art that was developed by Morihei Ueshiba as a method that integrated his Buddhist philosophy with nonlethal techniques from an array of traditional Japanese fighting arts.
Imi Lichtenfeld developed krav maga out of his experiences defending the Jewish quarter of Bratislava in the 1930s. This fighting art was designed specifically to be taught quickly to a large group of recruits. For this reason, most krav maga techniques use a small repertoire of movements as their foundation. For example, defenses against strikes are all relatively similar, whether the attacker is using his fists or a weapon.
Morihei Ueshiba developed aikido as a way to combine his belief in spiritual harmony with physical interactions in the form of self-defense and self-improvement. The main idea of any aikido technique is that you must effectively blend with and match the force of your opponent's attack and redirect it. For example, if an attacker punches at your head, you would catch his punch, move out of the way and redirect his punch into a throw.
Krav maga uses very direct movements that rely on stopping your opponent's attacks with counterattacks until the opponent is neutralized. Aikido relies extensively on circular movements intended to redirect your opponent's attack, allowing you to move around strikes and grabs. Krav maga is also intended to be learned quickly, so concepts are simple and consist purely of physical techniques. Aikido is intended to be spiritual as well as physical, and mastering the techniques may require years of practice.
Krav maga and aikido both emphasize the use of leverage and body position, so even if the practitioners are smaller than or not as strong as their opponents, this is not a severe disadvantage. If the attacker throws a punch at your head and stretches his arm out to his full reach, both aikido and krav maga will teach you to avoid the attack and use the attacker's extended position as an opening for defensive or counteroffensive techniques.