Physics - strange jumps.

Hello, guys! Here's the deal: I've got a 3d world with a 2d character in it (don't let the screenshots fool you, I've enabled 2d for better observation). There is a capsule collider on the character. Here's the code for it's controller:

 using UnityEngine;
 using System.Collections;
 
 public class JellyNew : MonoBehaviour {
     
     private GameObject child;
     public float maxSpeed = 5f; //Declare maximum speed
     bool facingRight = true;
     
     //Getting animator component from a child object
     public GameObject childWithAnimation; //Declaring input for editor
     Animator anim;
     void Awake () {
         anim = childWithAnimation.GetComponent<Animator> ();
     }
     
     //Ground Check
     bool onGround = false;
     public Transform groundCheck;
     float groundRadius = 0.2f;
     public LayerMask whatIsGround; //Declaring input for editor
     public float jumpForce = 700f; //Declare jumping force
     
     //bool doubleJump = false;
     
     // Use this for initialization
     void Start () 
     {
         child = GameObject.Find ("SpriteRotate"); //Finding a child which should be rotated
     }
     
     
     // Update is called once per frame
     void FixedUpdate () 
     {
         
         onGround = Physics.CheckSphere(groundCheck.position, groundRadius, whatIsGround);
         anim.SetBool("Ground", onGround);
 
         //if(onGround)
             //doubleJump = false;
         
         //How fast we're moving up or down
         anim.SetFloat("vSpeed", rigidbody.velocity.y);
         
         float move = Input.GetAxis ("Horizontal");
         
         anim.SetFloat ("Speed", Mathf.Abs(move)); //This is our current vertical speed
         
         rigidbody.velocity = new Vector2 (move * maxSpeed, rigidbody.velocity.y);
         
         if(move > 0 &&!facingRight)
             Flip();
         else if(move < 0 && facingRight)
             Flip();
     }
     
     void Update()
     {
         //If player on the ground and we're hitting 'Jump' button, we become no longer 'on the ground' and we're jumping
         if(onGround  && Input.GetButtonDown("Jump"))
         {
             anim.SetBool ("Ground", false);
             rigidbody.AddForce (new Vector2(0,jumpForce));
             
 
         }
     }
     
     void Flip() //This hideous, yet effective function rotates the child object with sprite and animation
     {
         facingRight =     !facingRight;
         Vector3 childScale = child.transform.localScale;
         childScale.x *= -1;
         child.transform.localScale = childScale;
     }
 }
 

When I stand on a step and jump, I jump just as I intend to - not too high up. That's the correct version. Here's a screen of a normal jump:

THE PROBLEM: When I run towards the stairs and jump off of one step's edge, the character jumps enormously high. You can see how high he jumps on this screen:

What's the problem with this all?