AtB opposition units will very frequently outnumber your force. Mostly because the assumption is that the player is much better than the bot. Additionally, very often, the opfor uses fairly crappy units (Vedettes, Scorpion tanks, etc) that just aren't as good as the hand-picked units a player will field.
That being said, there are several ways (in-game and settings) to manage the opfor size:
1) Set difficulty level to easiest or second easiest. This will greatly limit the number of enemy units deployed.
2) Uncheck the "double opfor vehicles" checkbox in the AtB tab, this will reduce the number of tanks the opfor fields.
3) Uncheck the 'allow enemy infantry' and 'allow enemy aeros' checkboxes in the AtB tab.
4) Make sure you're running at or close to the lance weight brackets. 130/200/280/380. If your lance weighs 135 tons, you'll be fighting opponents rolled from the opfor table for a 200-ton lance.
5) If you're still getting your clock cleaned, feel free to send extra guys in as reinforcements anyway or manually remove units once you get to Megamek, the bot won't mind.